How to change my IOS Back Ipad

Having updated your iPad, you cannot restore an earlier version of iOS/iPadOS than is currently installed.

All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple for a short period - however, usually within days, Apple will revoke the digital certificate with which the earlier version is signed. 

You cannot.

Apple has NO procedures NOR provisions to be able to downgrade/revert to any previous iOS/iPadOS upgrades/updates.

Sorry.