Upgrading NixOS so that it uses a different channel is simple.

When you run following command as root:

nix-channel --list

You should see a list of channels. If you did not touch channels so far, you should see this:

nixos https://nixos.org/channels/nixos-21.05

By running the following command (as root):

nix-channel --add https://nixos.org/channels/<channel version> nixos

you will remove the 21.05 channel and replace it with <channel version>.

To apply the change, you need to run following command:

nixos-rebuild switch --upgrade

which is going to rebuild your system with your current configuration and updated packages from the channel you switched to.