How to change Screen Time passcode

You'll need your existing ScreenTime Passcode to change it:

Settings > ScreenTime > Change Screen Time Passcode

If you have forgotten the ScreenTime passcode, the procedures outlined within this link should resolve the problem:

If you forgot your Screen Time passcode - Apple Support

If your iPad is capable of supporting iPadOS 13.x or later, you should update to the most current version before attempting to resolve a forgotten ScreenTime passcode. The most recent iPadOS updates have introduced a new mechanism whereby a forgotten ScreenTime passcode can be reset without wiping the iPad.

To reset a forgotten ScreenTime passcode on older versions of iOS/iPadOS, the iPad must be complete reset “as new”, losing all locally stored data:

Settings > General > Reset > Erase All Content & Settings

In this situation, you’ll not be able to restore your data from an iCloud or iTunes backup - as the forgotten passcode is integral to the backup.