The Linux Mint 21 distribution with long-term support (LTS) was released on August 1, 2022.
In mid-August I made the transition from 20.3 "Una" to 21 "Vanessa"..

Introduction

Unlike previous distributions, which were updated using the standard "do-release-upgrade" utility,
which appeared after the release of the new version of the OS, for the transition to Linux Mint 21, a new update tool "mintupdate" was created, with which the update is carried out with just a few clicks of the mouse in the graphical interface.

1) Preparing for the update. I recommend updating packages to the latest versions

sudo apt-get update
sudo apt-get full-upgrade

Then, before upgrading the OS, I recommend performing the following point number 2:

2) in the "Start" program - "Settings" - "Application sources" disable PPA repositories,
which you can do without at the time of transition. PPA repositories can be reconnected after upgrading to the Vanessa version.

After disabling the PPA, click OK in the window with the message "The configuration has been changed. Click OK to update the APT cache"

3) Installing the "mintupgrade" tool to upgrade Linux Mint:

sudo apt-get install mintupgrade

4) Launch "mintupgrade" to start updating the OS.

sudo mintupgrade

Welcome window:"Upgrade to Linux Mint 21 Vanessa":

5) I immediately recommend turning off snapshot creation - although this is a risky step, it saves a lot of time.

Disabling snapshot creation is done in the “hamburger menu” (a button with three horizontal stripes).
Disabled the item in the window "Recent Timeshift Snapshot"

6) Return to the previous window (click on the cross) and press the button "Let's go!"

7) Stage 1. Preparation

Click OK.

The APT cache is being checked.

If you cannot connect to one of the application sources,
the problem will be reported and you will be asked to disable this source,

for example: if "deb://librewolf.net" is not available. Click "Fix".

Other users may not have the specified problem, then the transition to the next step will occur.

8) B "Application sources" - "Mirrors" by default, the closest and fastest servers in Russia are selected for the previous distribution "20.04 una".
But they will be disabled during the update! The repositories will switch to servers during the Linux Mint update. on which the repository for Linux Mint 21 "Vanessa" is stored, i.e. foreign servers.

You need to click on the list "Basic (una)"- servers will be searched and ranked by speed.

I chose one of the main repositories with good access speed - in this case, the central "Linux Mint" repository in the USA.

Also clicked on the list "Basic (focal)" performed the same steps. The server selected is ubuntu.mirror.constant.com

After changing the mirrors, the following window view was obtained "Application sources" - "Official repositories" - "Mirrors":

Pressed the button OK to continue. The update of temporary files (cache) has begun.

9) In "Additional repositories" I unchecked all the boxes except Yandex Disk.

10) Clicked on the “Cross” in the title of the “Application Sources” window, the system update continued:

Searching for third-party packages:

A window was displayed indicating the presence of packages whose version numbers will be rolled back.
"Third-party packages. The following packages must be rolled back to the official versions"

Next, I scroll down the list (for reference):

Next, I scroll down the list (for reference):

11) Press the red button at the bottom "Fix":

After which, the package versions will be adjusted to the required ones.

Then, for the same purpose - bringing to the required versions, it will begin search for third party packages.

12) The next window will display a list of "Unused packages" , which are missing in NEW repositories (i.e. repositories "Vanessa":

I scroll down the list to see which packages are missing in the new version...

I press "Fix."

13)Stage 2: Simulate and download packages

You expect a long process, but almost immediately an error appears: "Your package cache could not be updated correctly. Run " apt update " and fix the errors that the command shows."

Following this advice, I launched the "Terminal" program and executed

sudo apt update

It is necessary to remove packages that the program will complain about. As usual - sudo apt remove

We repeat the process of checking and “targeted” removal of packages until this error disappears.

Check again

Finally, the program will stop finding errors and allow the update to continue:

Check again. No errors:

14) The process of simulating the update will begin.

15)After checking before updating, a detailed summary of upcoming changes to the system will be displayed: Download size 4.1 GB, Additional space required 3.8 GB. 3521 packages will be updated and 695 new packages will be added, 81 unnecessary packages (old versions, etc.) will be removed.

16) After clicking OK, the download of packages (4.1 GB) will begin.

You can watch the process of downloading packages

The download process is complete.

17)Stage 3. Update.

I click OK. The update process has begun.

Update completed successfully:

18)Reboot the PC.

OS loading successful.

19) Go to "Start" - "Settings" -"Update Manager".

For update package availability vkd3d we're not paying attention yet.

Necessary switch to local mirror!
The cache is updated again. We are updating new packages.

20) It remains to include PPA repositories and additional repositories for third-party programs in the “Application Sources”.

Conclusion: Updating Linux Mint 20.04 to version 21 "Vanessa" takes place in three stages:"Prepare", "Simulate" and "Update".

Tip for Upgrading to Linux Mint 21 "Vanessa"

If the process freezes or loops, simply interrupt it. Then run the same command

sudo mintupdate

The installation will continue from where it was interrupted. In my case, the interfering package turned out to be proftpd(the installation did not move forward). I had to remove the package: sudo apt-get remove proftpd. After the update, I installed and configured the proftpd package again.

What's new in Linux Mint 21 "Vanessa"?

1) Linux kernel 5.15:
_Linux 5.15.0-46-generic#49-Ubuntu SMP Thu Aug 4 18:03:25 UTC 2022 x86_64 x86_64 x8664 GNU/Linux
2) The remaining packages are close to the 22 LTS package base
3) Support until 2027
4) Added a new service systemd-oomd that monitors low memory situations
5) Updated Blueman component and bluez subsystem for Bluetooth audio
6) Improved support for the image standard for www - now WebP is shown in thumbnails in the Xapp program.

My experience using Linux Mint 21 "Vanessa"

The system differs little from 20.04 externally. Major changes “under the hood”: the process of using a SWAP file when there is a lack of RAM has become almost invisible. When there is insufficient RAM, GIMP stops crashing.
After downloading, in idle mode (without running programs) the RAM usage is 800 MB. The performance has not changed: on a dual-core Intel Core2 4300 (-MCP-) speed: 2160 MHz and an SSD drive, the Linux Mint 21 "Vanessa" operating system flies. The icons of the Remmina remote desktop program, for example, have changed and become more beautiful.

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