As both eluded to, you must first remove the hammer from the gun. Have the right size punch. Use Kroil penetrate to loosen up the pin, let soak for a few hours or overnight. Put the hammer on a bench block, (steel preferred), to drive out the pin, or an anvil with a hole to put the pin location over. If you do not feel competent to open the gun up and remove the hammer, it would be best to take the gun to a gunsmith.