Charlie's way is the cheapest, maybe the best.
Otherwise you have to cut around the edge where it is delaminated, lift
the piece out of the way, sand both sides (not absolutely necessary but helps bond), spread epoxy
, put the piece in place and weight down, and then later fix the cut you originally made.
You need to drill enough holes to fully get the resin everywhere and to get the edges so the delamination
stops. You can get empty caulking gun tubes that make injecting epoxy easier.
Use a good epoxy, don't use polyester resin. That's the problem now.