Often they are just a large head rivet, so the carpet doesn't pull up around the rivet. Start with a small drill like 1/8" and drill thru the rivet. Keep enlarging your drill until you find the right size that cuts the body dia. If there is a spot you can see the underside of the rivet, then use a drill bit just large enough to cut the body diameter then that head will pop off. Typical rivet diameters are 5/32, 3/16", and 1/4" dia body...