How to Reply to a Message in Outlook 2013
Type your reply in the Message box.
Don't be alarmed when you discover some text already in the text box — it's part of the message to which you're replying. Your blinking cursor is at the top of the screen, so anything that you type precedes the other person's message. (This arrangement means that the person who gets your message can review the original message as a memory-jogger when you get your reply.)