When using iOS 7, you can create a passcode to help protect your data and keep your kids from using your device without your permission. By default, when creating a passcode, a 4-digit numerical passcode is used. However, if you would like some additional security, a traditional password containing upper and lower-case letters, special characters, and numbers can be created.
Creating an Alphanumeric Passcode
Open the Settings app and select General. Scroll down and select Passcode Lock. Move the slider for Simple Passcode to the off position (no green is visible on the slider). This will allow you to create an alphanumeric passcode instead of a 4-digit code.