How to reset my screen time passcode on my old iPad ?

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.

It will need to be wiped clean, so hopefully it has a backup. There is no way to reset the Screen Time passcode when it has been forgotten. For more information on the lost Screen Time Passcode, you can review this page, but it will direct you to one of the other support articles below depending on the iPad OS version.

If you forgot your Screen Time passcode - Apple Support

If your iPad is using OS before 15.2, then you will have to follow this support article.

If you forgot your iPad passcode - Apple Support

If you are using OS 15.2 or greater, you can use this method with your Apple ID.

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

If following the linked process, you should be able to reset the iPad - but as noted, you must not restore your existing iCloud/iTunes backup, as the [unknown] ScreenTime Passcode will also be restored.