Rebuild and switch home manager:
make
Rebuild and Switch System
sudo make nixos
Note
Installing from the installer and rebuilding from the repo means systemd-boot may remain and load before grub.
Make sure to rm -rf /boot
and rebuild the system to make sure grub is the one loaded.
Go to /nix/var/nix/profiles/ and remove symlinks for generations you want to delete, then nix store gc.