If no one has answered or looked at it yet, and you are able to make a minor change quickly (spelling mistake, etc), I wouldn’t say anything. When I had to use the iPhone link to ask a question, I sometimes had to quickly make a change once I posted it, because the format wouldn’t allow me to preview my question. If I’m changing a fact, sentence or something that would make a person change their answer, I would add “edit” in bold letters either at the end of the question or to whatever part of the question the edit applies.