You're right..there aren't instruction to upgrade from v5.5! (this is the last version that won't help migration from a previous version...promised!!)
I hope that you still have your mGalaxy v5.5 folder available on your disk!?
In that case, your favorites aren't lost! You will have to copy paste theme from 'old' folder
example: "mGalaxy v5.5\Data\Multi-system [MAME]\favorites.ini"
to 'new' folder
example: "mGalaxy v6.0\Data\UserSystems\Multi-system [MAME]\favorites.ini"
For your keyboard shortcuts, you could copy/paste the codes themselves (not the entire line!) from
"mGalaxy v5.5\Data\Systems.xml"
to
"mGalaxy v6.0\Data\Settings.xml"
...but I discourage you to do so and in my opinion you'd better set it again with mGalaxy_Runway!