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.
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.