enid_puceflange
Get yourself a pin punch from a hardware store - a 1.5mm diameter one if my memory serves me - and then use anything reasonably heavy (I tend to use an adjustable spanner) to just gently tap the end of the punch until the trigger pin comes out.
Whilst the pin is out take the opportunity to clean and polish the pin, the holes it goes in and the trigger slot in the grip frame - this will give your trigger a smoother action.
You can use a nail, I've used one when some ******* stole my pin punch, but I'd flatten the point first to try and avoid any nasty scratches if you slip.
To put the pin back in, you can sometimes just initially push with your thumb, but to finish the job I'l lay a flat piece of wood - I use a wooden spoon - on the end of the pin and hammer that. This again avoids any nasty scratches or dents on the frame.