You'll probably get dozens of preferences on solder. You aren't talking about a strong bond, so I'd go with a lower melting point solder like Tix. However, there's probably plenty of solder already there, and it can be reused just fine. You need a very hot iron or high wattage solder gun. Heat from the other side (opposite the solder) while holding the retainer in place with a screwdriver or some other device. Then polish the brass if necessary. I'd advise against flame torches. They discolor the brass more.