If you have more money than time, there are kayak shops and other places that will replace it for you. It is not always the most pleasant thing to do, but I'm sure someone will likely disagree with that statement.
The next question you should ask is how to do it. I invert and secure my dry top over an appropriately sized kitchen pot. I then stretch and secure the gasket over the pot and rollback about an about inch such that the two glue surface are immediately adjacent. I apply just enough glue and then roll the folded back gasket back into place. I then put 2" clear packing tape precisely and neatly to hold the gasket and drytop perfectly together. Then, don't touch it, don't mess with it, walk away for a day and hope it all worked out. How do other people do it?
A few more hints. Don't use your SOs favorite kitchen pot. Do it in a room where the cats can't investigate.