Press studs are notoriously difficult to remove. Each part is made up of two halves, which are squeezed together using a special machine or hand-tool, with the fabric sandwiched in-between. However, I have had some success in removing studs from jeans and leather jackets using a hand-drill with a small diameter drill bit, say 1/16" or 3/32" dia.
Drill carefully through the centre of the stud with the garment held in a vice. If the stud is plastic, it will simply melt and disintegrate. If the stud is metal (most likely) it will require some effort, as well as care not to damage the surrounding material. Be careful ... metal studs can get very hot!
Eventually you wil drill through, and the 2 halves should fall apart. If they don't, a screwdriver may be used to prise the halves apart. Ensure that the replacement studs are larger than the hole left by those removed, otherwise there might not be enough material for the new press studs to grip.
Richard T.
May 2010