Most overlays are just stitched to the palm base, there are some however that are attached with adhesive before being sewn. A simple seam ripper should handle popping the stitches. If the overlay is attached under the binding (as most are) that would require very careful cutting of the overlay to get it off.
It’s repairable, but not necessarily inexpensive. To do that job correctly the thumb needs to be taken apart, the ripped top cover repaired and then re assembled. The binding would also be replaced. Those type jobs normally run in the $25-30 range.