After removing the slide from the frame (a step which may be safely omitted), I now focus on the magazine release. The USP's magazine release is the small ridged piece of plastic behind the trigger, near the roll pin going through the grip. As you can see, there's not a lot of surface available on the stock mag release for your thumb to engage on when you try to release the magazine.
In order to avoid marring the finish, I used a brass punch. Brass is softer than the steel roll pin, hence ensuring that it won't mar the pin. This is me lining it up with the top of the pin. Note the spring in the center of the magazine release.
Using a brass hammer to gently drive the pin through the grip to the other side. Once it had moved some, I shifted the gun so that I was striking the pin in from the side.
Sending the roll pin out the rest of the way. Had I kept the gun positioned as in the previous picture, the pin would've been driven into the wooden sheet these pictures were taken over.
Once the roll pin has been driven through and is out of the frame, your punch will hold the magazine release in place exactly like the roll pin used to. This is convenient, as it ensures that the spring doesn't send the mag release flying as soon as the pin's out of the way.
The extended (left) and normal (right) magazine releases, side by side. Make sure to transfer the spring from the original one into the one you intend to install.
Same two releases, different angle. This angle makes it a bit more apparent how much room there is for the thumb to contact the release.
The USP, disassembled, including the mag release.
Loosely putting the mag release in place. Note the direction of the spring, and that the notch in the front lines up perfectly with the trigger guard when viewed from below.
Another angle on the installation of the magazine release. Note that there are several ridges which the spring tries to hang up on.
I chose to carefully use a knife blade to keep the spring off of the ridges, so that it could slide into the channel it needs to be in. As the underside of the trigger guard suggests, you should probably refer to the owner's manual before using steel knives to work on your polymer gun. A screwdriver is probably the best choice here.
The entire assembly fits together well, even without the roll pin installed. Note how the extended magazine release extends out farther on either side, than the factory release.
Installing the roll pin. First you line it up over the hole, then you gently hammer it in using the brass punch.
That's it - the extended magazine release has been installed, with no more than a punch, a hammer, and a pocket knife (or screwdriver). -- SeanNewton - 03 Sep 2012