I picked up a rod that has a garrison style cork grip and seat with a cap and ring. Said rod was stored with a reel attached and the ring left a depression in the cork. How does one go about fixing that?

Tom already posted a great link. I thought I might add a similar approach for small indentations (like from a guide hitting the grip or in another case a dog biting a grip of mine ),is to place a wet washcloth over the dent, and place the tip of an iron for a second or two on the washcloth where the dent is located. Repeat until the dent is gone. I generally use a medium setting on the iron. Good luck