Easy as pie. Lower the hammer, take off the grip panels, press the lanyard loop, which is actually part of the mainspring cap, down on a hard surface so that it is pushed in just a slight amount (this takes mainspring pressure off of the retaining pin) and then push out the retaining pin. The retaining spring can be pushed easily. It doesn't need to be driven with a punch. If it does, you're doing something wrong! Take out the old spring, put the new one over the hammer strut, and put the cap back in. Takes more time to describe than it does to do.
You have several choices in springs. The 92D spring is good, and some people on Beretta-L report even better results with the spring from the 8000F Cougars. Supposedly it stacks less. Do not try to use the Cougar D spring, as you will get light strikes constantly. Another thing that works is a standard 1911 mainspring. I'm using one right now. Works great, especially if you happen to have one lying around like I did.