You'd have to get new foam for the inside. I'm not sure where you could get it though. The Fabric i think you can slip off with some effort and even more effort putting it back on. You may also be able to take it to an auto upholstery place and have them replace the foam on the inside of the seat(s). Good luck!
------------------
Mustang Parts Specialties ripped me off, don't buy from them.
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT