@tomasoqvist If you would like to stay in pmos stable as in stable then there are no recommended methods.

But because linux mobile is evolving rapidly fast my suggestion is to go for the edge.

This is not recommended but you could add edge mirrors to /etc/apk/repositories and then run sudo apk upgrade -a

https://wiki.postmarketos.org/wiki/Mirrors

I have done it many times and for me it did no harm at all. Backup might be wise before doing it still :).