I did it the lazy way. I installed downgrade package from the repos which is meant for downgrading packages but it also let me upgrade as well
I’m using pamac and I don’t know how to do what you said, but I updated the glib package thought another method:
sudo pacman -U http://ftp.riken.jp/Linux/manjaro/unstable/core/x86_64/glib2-2.66.7-1-x86_64.pkg.tar.zst
where the url is some mirror to the unstable package.
I hope this won’t break anything and the package gets updated automatically on futures manjaro updates…
Edit: the update to glib2-2.66.7-1 worked too Thanks!