You do not understand.
You downgrade your iPad, you will not be able to retain the third party apps you may already have on that iPad.
Virtually all third party app developers have abandoned all 8-10-year old iPad models running iOS 9/10.
Older iOS 9/10 compatible apps are few and far between, now.
Virtually all gone.
You may very well render that iPad useless.
There is one last thing you can try/attempt, if you need an older version of an app.
If you own much newer iOS device that can run either iOS 12 and.or iOS/iPadOS 13 and later, you can install the most recent app/s onto that more recent iOS device.
Then go back to your older iPad, open the iOS App Store, go to the Purchases section of the store app, search for the app/s you wish to try and download to that older iPad and see if an older version of the app is allowed to be downloaded/installed to that older iPad.
These newer iOS/iPadOS devices must use your same Apple ID user account and password as the older device you ard trying to get an older app for.
Failing that OR IF NO joy, then there is nothing more to be done.
What you are wishing to attempt is overall a very bad idea.