LMDE 6 has only been out for less than a year. It's based on Debian 12. Debian 13 will likely be out around this time next year, and the future LMDE 7 will be based on that.
Mint for years but i forget. In the updates make sure fully updated first. Then i remember some sort of upgrade command.
Im not much help but i think i did it once. Or you can preserve your home folder, broweser bookmarks, etc… the load the new os from scratch, but im sure theres an upgrade option.
The update functionality for older versions to upgrade to the new version will be released a few weeks after the iso release of Linux Mint 22. It’ll be an available upgrade in the Software Update Center application.