You can read the documentation here: Manjaro Immutable Out Now for Community Testing
sudo bootctl set-default <and the shasum of the image>
latest might have e37c0525b4cf18ddd2fe29fc360dd91f7d6aac55b4
as in 6.9.12-1-2024.08.03-1 release in testing branch.
you can check cat /arkdep/tracker
or do a mount | grep arkdep
to get the shasum.