As far as I can tell, unless you're renewing and upgrading at the same time, there's no way. So if it's coming up on renewal time and you want your next AP to be at a higher level, you can do that (and they've been giving you the time left on the first AP at that higher level, which is cool). But I can't find a way to just upgrade it online.
Whenever your next trip is, just go to the Guest Relations/Services window (one is to the left of DL, one is to the left of DCA...it's possible the one near DCA can't do it, I don't know, but we dealt with AP stuff at the one next to DL) and see if they can handle it there.