Extracted from s/n2: https://easylinuxtipsproject.blogspot.com/p/upgrade-mint.html#ID2

For easy and quick ref. There are also other tips in the link.

a. Launch a terminal window.

b. In sources list, replace the Ulyana source (20) by the Ulyssa source (20.1).
For that, use copy/paste to transfer this line into the terminal (it's one line):
sudo sed -i 's/ulyana/ulyssa/g' /etc/apt/sources.list.d/official-package-repositories.list

c. Update the packages list:
sudo apt-get update

d. Perform the actual upgrade and wait for a while:
sudo apt-get dist-upgrade

e. Reboot computer once dist-upgrade is done.

Note from website that did NOT occur to me:
This procedure (replacing the nickname of the current Mint by the nickname of the new Mint in the sources list) will also be the same in the future, up to and including the future final member of the Mint 20 series, Mint 20.3. Unlike upgrading by Update Manager, this terminal upgrade will also install the recommended kernel for the new Mint, even if it would be one from a newer kernel series.

If your hardware doesn't run well on such a newer kernel, you can simply boot from the former kernel (it's still there, hidden under "Advanced options" in the Grub bootloader menu) and remove the latest kernel. You can easily remove a kernel by clicking View - Linux kernels in the panel of Update Manager.