Gizmo went into more detail than I did.
But anyway you would change the charset in the language file back to UTF-8
Run the tool in the UBB Control Panel to convert to utf8mb4 which will change the tables.
Then you may need to rebuild tables. run the re builders and or run some update queries.
You just need to make database backups in case it fails for each step.
Also there are lots of posts about this feature here.
There is no simple way to undo this so that is why you need backups of the database.
