Ouch. Where to start? First, you can't combine UA and US miles, and even if you could it takes a minimum of 30,000 miles [B]each way[B] to upgrade....and that's on a higher fare class than I bet you have. The outbound trip to MUC is on UA, and looking at the seating chart there are quite a few seats available in the Economy Plus section, so if you are lucky you might be able to pay a fee on the day of your flight for E+, which gives you about 5" of extra leg room.

Your return is actually on a Lufthansa flight (you can tell by the 4-digit flight number) so there is no E+. I find Lufthansa economy seats to be extremely uncomfortable, but short of paying for an upgrade to Business I'm not sure what you can do. If you could change your flight to a UA product you'd have the same opportunity to (maybe) buy up to E+. All in all..... it sounds like you're in for a fairly uncomfortable few hours, but hey....you're getting a free trip to Europe!