how to install an old version of IOS on my iPad

The ability to restore an earlier version of iOS/iPadOS is limited - but is possible in certain circumstances. The procedure is not trivial.

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 - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed. After a major update, such as the initial release of iPadOS15, the period that the preceding version is signed my be longer.

Reversion to an earlier version will entail the use of “DFU” mode to reinstall the previous version; all locally stored data will be wiped. If you have an existing iCloud or iTunes backup of your data, your data can be restored after reinstalling the previous version of iOS/iPadOS. You’ll need access to a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS).

Be aware, that you can only restore an iCloud backup that was made for the version that you have reinstalled - or to a later version of the OS. As such, if intending to restore iPadOS14.8, you’ll not be able to install a backup that was made using iPadOS15.

So as to avoid unnecessary description of the process, iMore have a useful article that describes the procedure of restoring an earlier version. The article refers to a different versions of iOS, but the process is the same.

https://www.imore.com/how-to-downgrade-ios

After successful reinstalled of an earlier version, if you have an appropriate backup, you’ll be able to restore your backup to your iPad:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

A little hard to help/advise you when you provide NO information about your iPad.

Here are my questions to you.

Can you, please, confirm what year and model iPad do you have/own?

The “A” model number can be found on the aluminium back of your iPad, in very small print, right under the larger bold print “iPad” on the back of the iPad.

What iOS/iPadOS version is this iPad, currently, running?

What is the rated, internal data storage space/capacity of your iPad model and how much of this free, internal data storage capacity is left inside of your iPad model?

All of that additional info can be found in the Settings app, General settings panel, About.

Please try to attempt to answer all questions asked.

Thank You.

What version of iOS/iPadOS is your iPad currently running?

I asked you a list of questions.

Please atempt to answer them

Is this iPad working and running?