Short and sweet and excellent advice from Alex.
1) Remain calm – We all know how awkward it can be, but just stay calm. If you get upset or react in a certain way then the other person may also get upset by this.
2) Apologise and correct yourself – Make sure you keep it short and sweet. It’s good manners and it highlights that you have made a mistaken and that you are truly sorry.
3) Move on – Being misgendered is very uncomfortable but it always happens, it can happen to anybody. Make sure you avoid making it into a big deal, Just apologise and carry on the conversation like nothing happened.