Unless you're on a widebody with a disability restroom (those usually have nice-sized changing tables) your best bet is normally the floor directly outside the lav in the tail of the plane, across from the galley in the open space by the rear door.
If possible, position yourself so that you are holding the lav door open with your foot, so that you can reach in to dispose of nasties without stepping away from your child. Pick up your change pad, and if there is someone back at your seat, carry baby back and then return to completely wash up. [Also, carry some plastic bags for disposing of smelly dipes in the trash, that way the smell will be easy to quickly contain.)
I don't suggest the lav floor for the same reason that I don't suggest going barefoot on it. Baby would be within arms reach of a lot of possible bad-aim-debris, and that is really hard to police while trying to quickly change a diaper.
Whatever you do, do NOT change the diaper at your seat, unless you are dealing with a fastened-seat-belt-sign *and* an impending leak.