The first stage of field stripping the USP: press the slide backwards until the notch on the lower part of the slide (just below and left of the 'AK' engraved on the slide in the picture) is right above the center of the slide release lever.
Press the slide release lever from the other side of the frame, and it will come out. Once you've removed the lever from the frame, the slide is unlocked and you can now press the slide forward, removing it from the weapon.
Push the slide forward, off of the frame. Notice that while the frame itself is polymer, the slide makes contact with metal protrusions embedded in the sides of the polymer frame. Those provide contact points between frame and slide, thus reducing metal-on-polymer abrasion as the weapon cycles.
This is the slide, shown from below. The spring you see is the recoil assembly, which locks together with the barrel.
Removing the barrel from the slide. Note the slot towards the rear of the barrel, which interlocks into a machined piece at the rear of the recoil spring assembly.
The USP, still field stripped but with the new magazine release installed. Note how the barrel, above, interlocks with the guide rod assembly.
The USP is now field stripped. Reassembly is simply a matter of reversing the assembly steps. While it's apart, you may find the points below to be noteworthy. The recoil spring assembly slides into a notch on the frame down below, and is thus captured within a channel on the frame. This is an interesting contrast with the CZ and Sig pistols I've seen. Less remarkable is the plastic buffer.
-- SeanNewton - 03 Sep 2012