If they're the ones I'm thinking of, they're kind of like reusable pop
rivets.  you need to pull the center out of them, then withdraw the
whole thing.  If they're really munged up, I'd probably set some rivet
nuts in the bumper and use small machine screws.

something like this:

http://www.rivetsinstock.com/rivet50.htm

if you only need a couple you don't need an insertion tool, you simply
tap them into a hole, take a machine screw the correct size/pitch,
thread a nut and washer onto the machine screw, thread that whole mess
into the rivet nut, hold the head of the machine screw still while
running the nut away from the head with a wrench.  That takes a little
longer than using the right tool, but then you don't need to buy the
tool if you're only inserting a couple fasteners.  I like having them
around because they're also handy when you have a hole through thick
sheet metal that originally had a machine-type screw through it, and
the hole has been worn out/pulled over the years (e.g. coil bracket on
the firewall of an old Studebaker, or lots of window regulator
applications.  I also used them on my screen door when the pneumatic
closer ripped itself off of it...)

Another alternative would be to check with your local dealer, the
"correct" fasteners might not be all that expensive.

good luck,

nate