Easy but frightening. It is slow and for me hair-raising at times, but I've leaned to have faith in every single Linux thing, most of the time. Half way through it lost the Internet, couldn't find all the packages it needed to get rid of, and just sat there staring at me. So I did the only sensible thing I could think of and went away while it did nothing - no flashing red light on the computer case.
When I came back after awhile there was a sign on the desktop which guided me. Everything worked, but I avoided going straight through to 21.3 because there is a warning that it might leave the computer unbootable. Probably rare, but......
Great little gizmo they've made for the upgrade, but be prepared to let it work:
https://linuxmint-user-guide.readthedocs.io/en/latest/upgrade-to-mint-21.html
