How to add space IN iPad AIR 2 ?

It isn’t possible to upgrade or extend the internal storage of your iPad. The internal storage configuration of your iPad is fixed during manufacture - and will be the same for the life of the iPad. You can, however, subscribe to a higher tier of iCloud storage - but this is not a substitute for internal storage.

If your internal storage is full, your only option is to better manage the storage that you have - and delete unused Apps or unneeded data. 

These support pages should prove to be helpful:

About storage on your device and in iCloud – Apple Support

What's the difference between device storage and iCloud storage? – Apple Support

Manage your iCloud storage – Apple Support

How to check the storage on your iPhone, iPad and iPod touch – Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

“Offloading” Apps can be a helpful method of creating temporary space for the software update.

https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/

Your storage is a fixed size and cannot be increased.

You will need to delete apps, photos, videos to gain back more storage space.

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