how to change pass code

If you've forgotten the lock screen passcode, the following has instructions for all of the methods of dealing with that: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

If you know the code and want to change it, go to Settings > Touch ID & Passcode (or Settings > Passcode) and enter the current code and then tap "Change Passcode."