Will depend on the make of derringer. They are not all the same. Would suggest you find a gunsmith to do this one. It is cheaper than taking one to them AFTER you work on it.
Take it to a gunsmith. Remove cylinder - remove firing pin pin located under rear site - use finishing nail/small center punch ,place against back of firing pin and tap lightly driving pin toward "removed" cylinder
To remove the firing pin from a Savage 440A shotgun, first ensure the firearm is unloaded and safe to handle. Remove the stock by unscrewing any screws that hold it in place, and then take off the trigger assembly. Next, locate the firing pin assembly, which is usually secured by a retaining pin or screw; remove this fastener, and then gently pull the firing pin out from the bolt assembly. Always refer to the manufacturer's instructions or a gunsmith for detailed guidance if uncertain.
You need a gunsmith
Best left to a gunsmith
Get help from a gunsmith
Best left to a gunmsith.
Find a good gunsmith or machinist
google it.
Remove the firing pin from the rear and the locking bolt will side out the side of the bolt
A firing pin that is mounted on/in a hammer
This would be best left to a qualified gunsmith.
Firing Pin Block