Assuming that you have the bolt assembly out of the receiver, here is what you need to do.
Observe that at the rear of the bolt assembly, there is a small pin. Punch this pin out from the small diameter end toward the big diameter end. This will allow the firing pin and the firing pin return spring to slide out the rear of the bolt assembly.
Replace the broken firing pin with a new one. Put the spring around the new pin. Insert it back into the bolt assembly. Replace the pin which holds the firing pin and spring in place by inserting the small end of the pin into the big end of the opening and pushing it through.