Oh are YOU getting ready to have some fun.
I'll tell you how it goes so you can learn from my mistakes and to save you some grief. First of all, as you know, compared to a 1911, removal is a pain in the a**. Second, since the firing pin hole of the USP is very small, surprisingly little dirt gets in there. When I cleaned mine after 6000+ rounds, it barely dirtied one patch. Your results may vary, of course. Still convinced you want to do it? Okay, here goes (it helps to have three hands for this). I assume you've driven out the pressed in drift pin just in front of the rotating retaining pin? If so, then, 1) Make a detailed note of the position of the firing pin and the retaining pin. I guarantee you will not remember their respective positions (once you have them out you will see why). 2) push in the firing pin with something about 1/3 of the way and hold it there while pushing in the retaining pin and rotating it. Run out of hands yet? The pin should come out, reluctantly, as does the retaining pin, which is also under spring pressure. Installation requires some fidgeting. It helps to have a pair of tweezers in this process as you have to hold the firing pin in again. Work with it. You'll get it eventually. Good luck