Clem has released the upgrade instructions for those wishing to upgrade their version of Mint to version 21.
If your running Version 20.3 go Here.
If an older version of Mint go here:
How to upgrade to Linux Mint 21 — Linux Mint User Guide documentationlinuxmint-user-guide.readthedocs.io
My personal preference is to do a complete fresh install of a new major version. But to each their own.