A passcode isn't the only security precaution you can take for your smartphone, but it's a pretty good safeguard against someone firing up your mobile device and seeing what secrets you have stored there. Prior to iOS 9, Apple used a four-digit PIN to keep out unwelcome visitors. iOS 9 ups that protection to a six-digit passcode
There's more significance to that change than adding just a pair of digits would suggest. Creating a six-digit passcode means that there are now 1 million different numerical combinations instead of 10,000, so even a brute force attack would take days instead of hours to unlock your phone. And even if tapping two extra digits to unlock your phone strikes you as burdensome, people with iPhones that support Touch ID — the iPhone 5s and later devices including the iPhone SE — will do most of their unlocking via a thumbprint anyhow.
(And if you think six digits don't provide enough protection, iOS 9 retains a feature from previous versions of the OS to customize your own passcode using a longer string of numbers or letters and numbers. That option's available whenever you change your passcode, as we'll see below.)
When you get a new iPhone running iOS 9, you'll be prompted to set up a six-digit passcode. But if you're upgrading to iOS on an existing phone, you'll have to make the switch to six digits yourself. Here's how to do it.
1. Go to Settings.
2. Select Touch ID & Passcode. (On iPhones without Touch ID support, this option will simply say "Passcode.") You'll be prompted to enter your current passcode to proceed.
3. Tap Change Passcode. Again, you'll have to enter in your current passcode.
4. Enter a new passcode. By default, iOS 9 will ask you to enter six digits. Tap Passcode Options if you want to customize your passcode with numbers and letters or stick with a four-digit PIN.
5. Verify your new passcode by re-typing it.
6. Decide if you want to sync up your passcode with the iCloud Security Code, which Apple uses when authorizing additional devices to use your iCloud Keychain. If you tap yes, you'll need to enter in your Apple ID password.
Should you decide that the improved security of a six-digit passcode just isn't for you, you can always return to the world of four digits simply by following the same steps listed above and selecting Passcode Options.