Find the nearest bathroom and change in one of the stalls. If the handicapped stall is open, use it as the extra space makes changing and cleanup easier. IF there is a family restroom, then use that instead, they offer much more privacy and space for changing.
For disposal from in a stall, roll up the used diaper and take it in hand to the nearest trash. Rolling it up in itself will seal in any bad smells long enoug till the trash gets taken out, so no extra bags are necessary. If in a family room, do the same.
Don't worry about being found out, outed, or even confronted about it either. Some people might probably stare or do a double take, but most by far never say a thing or bat an eye. Any embarrassment you might feel about it is really just in your own head. The more often you do it, the less you'll feel that way.