I think it's up to you, but if you correct a post you should always make it explicit that you have done so and say what you did. Like Lumifer, I think it's better to add a note at the end (or, in extreme cases, at the beginning) than just to overwrite the old content with the new; otherwise you risk making other people's comments nonsensical.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.