[Not a mod says] All questions and answers, once posted, leave you a maximum of 10 minutes to edit. During the time limit you can click on “edit you question/answer” to edit them. When the time limit is up, the button will disappear, and there’s nothing you can do about it by yourself.
For questions, you can flag it so that the mods can push them back for you. For answer, there is almost nothing you can do. You can write another response saying “Edit: <what you want to edit>”, or flag the answers like others have suggested.
Next time when you want to edit something, do it within 10 minutes.