I didn't even know UA booked Deutsche Bahn train connections; I thought the only code-share train service they booked was on the French TGV; I tried a dummy booking on the bomb and all that came up were air connections on LH. If they just booked a ticket for you with no discount, you should be able to change the return reservation with DB directly in FRA. The Deutsche Bahn website is www.bahn.de

And, as a friendly aside, there's no need to start a second thread if you don't get an answer right away -- just bump the original one if it drops off the front page.

I hope this was somewhat helpful.