Manjaro is a rolling release.
You don’t go back. Gnome 3.38 is the past, you cannot keep it.
There are extensions and layouts to get a seemingly 3.38 behaviour though: You can try first No overview at start-up - GNOME Shell Extensions to deactivate the launch on overview by default.
This thread has got some tweaks you can try to revert back - somehow - the visual appearance:
In Layouts, you can also switch to Manjaro Legacy to get back some settings you were used to (I haven’t tried though, Gnome way is to adapt to Gnome changes).