I need to change screen time passcode

If you have forgotten the ScreenTime passcode, one of the two 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, you should update to the most current version before attempting to resolve a forgotten ScreenTime passcode. The most recent iPadOS updates have introduced a mechanism whereby a forgotten ScreenTime passcode can be reset without wiping the iPad.

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

See this article.

If you forgot your Screen Time passcode - Apple Support