As I recall, it was a simple procedure. I'm doing this from memory, without a USP in front of me. After removing the slide, use a small punch to drive the roll pin out (holds in the extractor). It helps to apply a little pressure to the mid/rear of the extractor while driving out the roll pin, because the small spring at the rear of the extractor will shoot out when you remove your punch. The roll pin doesn't necessarily need to come completely out of the slide to free up the extractor. That will save you some headaches during reassembly.
I would also use this opportunity, once the extractor is out, to polish the breech face. I used 1500 grit paper on a popsicle stick and a felt bob on a dremel (with white rouge or Kit scratch out).
To replace, drop the extractor spring back in the hole and place the extractor back in the slot. Again apply sideward pressure to the mid/rear of the extractor to line up the holes, and drive the roll pin home. Make sure none of the roll pin protrudes from the bottom (internal) portion of the slide.
Such is the glory of external extractors. Now that I am learning a little about 1911's, I can appreciate the simplicity of external extractors.