For example, when ordering something, if you ask for one, but then think again and want to order two, you would say "One, scratch that, two." The closest I found is いや, but that appears to be casual/rude.
If it were me, a typical 外国人, I would say, "あっ、すみません。①ではなくて、②お願いします。" I think it's simple and with the all-encompassing "excuse me"/"sorry" word, すみません, the expression becomes more polite.
