The new operating system will now automatically default to a six digit PIN to unlock your device, instead of just a four digit PIN. While this might seem like a small change, it actually makes breaking into your iPhone a lot more difficult.
With a four digit PIN, there are a possible 10,000 combinations. But with a six digit code, there are 1 million possible combos, making it a lot tougher for someone to crack your security code.

- Launch the Settings app.
- Find and select Touch ID & Passcode.
- Enter your current PIN code if/when prompted.
- Find and select Change Passcode.
- Enter your current code again.
- Enter a new, six-digit code (twice).
You're done. Press the home button to exit the Settings app. The next time you're prompted for a passcode, remember to enter all six numbers.
from Apple Tech Insider / Cnet.com