 And it will tell you what to do. Okay, here it is. It explains the passcode. A passcode is like a numerical pen. It's usually four digits. It's different from the password. And it's one of the security measures for AT&T customers. How do I update my passcode? We've already reset your passcode. If you haven't changed it in the past year, we suggest you change yours as a precaution. You can sign into your account.