Before iOS 9, users had only one choice of setting a passcode with 4 digits. But now, you get to set a 6-digit passcode on iPhone to boost your security and decrease the chance of random password guessing. To create a 6-digit passcode on iPhone devices running iOS 9 or later, do as follows:
1. Open the Settings application
2. Tap to open the Passcode settings
3. If Passcode is already enabled enter your passcode, otherwise the Passcode settings will open
4. Tap the Turn Passcode On option (for those who already have a passcode, tap the Change Passcode option and type your old passcode in the opened page)
5. Before entering a New Passcode, tap the Passcode Options right above the numpad
6. From available options to choose the passcode type, tap the 6-digit Numberic Code.
7. And now, enter your 6-digit passcode, re-enter to confirm, and your are all Done and Set.