By now most iPhone, iPad and iPod touch users are probably aware that you can configure a four-digit passcode on your device to protect it from unauthorized access. For more security-conscious users, however, iOS also provides the ability to use a longer, alphanumeric passcode. To set this up, simply go into the Settings app, and select General, Passcode Lock and toggle OFF the Simple Passcode option. When entering a new passcode, you will be presented with the standard iOS alphanumeric keyboard instead of the simple numeric PIN pad, both for setting your passcode and for unlocking it when you access your device.
It’s also worth mentioning that information that iOS encrypts using the Data Protection APIs—such as your other passwords for things like Mail accounts and Wi-Fi networks—are encrypted in the iOS keychain using this passcode, so a longer or more complex passcode provides better security for this data as well, even if a thief or hacker bypasses your passcode by jailbreaking your device.
Note that if you’re using your device in a corporate environment these settings may be locked down by your IT department. Other related policies can also be enforced in this manner such as minimum password length and a requirement to change your password after a certain number of days.