6-digit Passcode

Use a passcode with your iPhone, iPad, or iPod touch - Apple Support contains those instructions, but if you forgot your passcode you will have to use the instructions provided in: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

See the following from: Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Change your passcode or passcode settings

On iPhone X and later, go to Settings > Face ID & Passcode. On earlier iPhone models, go to Settings > Touch ID & Passcode. On devices without Touch ID, go to Settings > Passcode.

and

Change Passcode: Enter a new six-digit passcode. Or tap Passcode Options to switch to a four-digit numeric code, a custom numeric code, or a custom alphanumeric code.