The easiest way to prevent unauthorized users from accessing the data stored on your iPhone, iPad and iPod Touch is to enable Touch ID (when available) or set up a passcode. The new iOS 9, update now offers a more advanced level of protection, allowing users to set customized codes with 4-digit, 6-digit, or custom length options.
Customize Your iPhone Passcode
Go to Settings > Touch ID & Passcode and tap on Turn Passcode On to access the Set Passcode screen. Tap on Passcode Options and then choose the type of protection you want to use.
The Custom Alphanumeric Code allows you to create a password consisting of letters (uppercase and/or lowercase), numbers, and other symbols (no length restriction).
The Custom Numeric Code option allows you to create a passcode made up of numbers (no length restriction).
The 6-Digit Number Code and 4-Digit Number Coded options are more basic, allowing you to set either 4 or 6-digit passcodes.