I've done several. You need a product called Solder Wick which you lay across the old joint and heat it with an iron. The old solder melts and flows up into the wick...done. Desolder all the joints on the old relay, pull the old relay off the board and plug the new relay into the holes. Resolder the new relay in place and you're done.
Guy B. Young of the COG forum used to sell these relays. YOu could also Google the part number and find them on the web for 2-3 bucks each. If you're not sure of your skills just send your box to Larry Buck.