First, get the old reel seat off.
The reel seat is probably held on by some sort of glue or epoxy. Hopefully, it's not too stout.
Boil a pot of water and put the butt end of the rod in a heavy plastic bag.
Dip the bag into the boiling water to weaken the glue. You should be able to pull the reel seat off.
It may take a little time, depending on the glue/epoxy used. Some good gloves will help.
While the glue is still soft on the rod blank, scrape it off the best you can. Use a plastic scraper to prevent damaging the rod blank.
To replace the reel seat:
It's a lot easier if you find a reel seat with the same dimensions.
Use a two part eopxy to put the new on on.....like rod bond or somthing similar.
Be sure to line up where the reel attaches to the rest of the rod.
The inside of the reel seat will likely be larger than the diameter of the blank where it fits.
Build up some arbors with strips of tape on the blank to make the reel seat fit even and snug, then do the epoxy work.