The "how to" part should be pretty easy. If it is built like most over-unders you will see the pins that hold in the firing pins once your remove the buttstock. They shouldn't be hard to get out. Make sure when you put them back in that the slide freely over the retaining slot in the pin. I had one in recently that someone had tried to drive the pin into when the slot wasn't lined up and marred the pin. The pin would then stuck from the bur it caused.
As for firing pins, e-gunparts has always been my go-to when I can't find the parts I need at a Canadian supplier.
If you need to get pins made, PM me. I can make you new ones with the old ones as templates. If you want to send the whole receiver to me I can do the whole job for you.