put a newer backup onto an older version of ipados

Update steps

Update your iOS device.        

Get help with over-the-air iOS updates          

Update – Can’t or can’t restore 

Have you tried using iTunes on a computer or Finder if running Catalina to do the update?

 Update software on iOS devices in iTunes on Mac

If you see an error when you update or restore your iPhone, iPad, or iPod      

Is the iPad with 13.5 compatible with IOS 14? You should upgrade the IOS before trying to restore the backup.

Yes. you should update your Gen. 7 iPad first then restore from the backup.