Nixos How to change Kernel branch

First update the nix channel.

nix-channel --update

Then edit your configuration.nix and add the following line to select your kernel branch (the kernel branch has to be in the https://search.nixos.org/packages?channel=25.11&query=linux+kernel)

# Kernel
  boot.kernelPackages = pkgs.linuxPackages_7_0;  

I’ve selected the 7.0 branch which is the stable branch for now according to kernel.org. And in the end update your nixos and reboot :

nixos-rebuild boot
sudo reboot