Apple started promoting the use of six-digit passcode with iOS 9. If you missed or skipped the iOS prompt to upgrade from four-digit to six-digit passcode, it’s wise to spend few minutes to switch to use six-digit passcode.
iOS has an option where device will be wiped clean after 10 failed PIN attempts. This safeguard however has proven to be hackable. With access to iPhone’s internal circuit board, a hacker can cut off power supply after each PIN failure. iOS will then not count the failed attempt, allowing the hacker to try out all combinations of the PIN. A longer passcode makes it much harder to crack.
Go to Settings > Touch ID & Passcode. Enter your current passcode to proceed.Tap Change Passcode. You will be prompted to enter your old passcode again. And then iOS will prompt you for a six-digit code. Enter your six-digit code and verify it to complete the change.