How to upgrade Nixos

First list the channel you are currently in :

root@Frigg:/home/mathieu]# nix-channel --list
nixos https://channels.nixos.org/nixos-25.11

Add the channel that you want to join. You can find channel list at this adress : https://status.nixos.org/

[root@Frigg:/home/mathieu]# nix-channel --add https://channels.nixos.org/nixos-26.05 nixos

[root@Frigg:/home/mathieu]# nix-channel --list
nixos https://channels.nixos.org/nixos-26.05

Update the local channel.

[root@Frigg:/home/mathieu]# nix-channel --update
unpacking 1 channels...

And update the system. There will be errors as there are differences between branches, if so correct the errors and repeat the rebuild step :

[root@Frigg:/home/mathieu]# nixos-rebuild switch