How to delete an email address

Your question could use a little more clarity, but I assume that you mean an email address that appears in the autosuggestion window when you start typing an email address in the To: field. If that's what you are asking about, when you see the address that you want to delete, tap the "I" to the right of the email address. Then swipe down to the bottom of the next window and tap Remove From Recents.