If you wish to convert your existing install to Niri, you can do this by doing:
pikman update && pikman install pika-niri-desktop-minimal pika-niri-settings
or for Nvidia by doing:
pikman update && pikman install pika-niri-desktop-minimal pika-niri-settings-nvidia
Please be aware if doing this you need to make sure you have an empty
~/.config/nirifolder and if you currently use Hyprland you will need to uninstall it's settings package or you will overwrite your settings with the defaults.