Drill out the old one from behind the target. You can reuse the small washer (if it has one).
Your problem will be a rivet press (or lack of one). You can buy the DIY bits from a tool shop and then you just hammer them in. Hard to track down, but they look like this ;
http://www.pinrestore.com/Riveting.html"Rivet Rollover Punch"
These guys used to sell a kit
http://www.pinbits.com/But they don't seem to be anymore.
