how to remove apple ID from ipad air

By entering the AppleID and passcode used to initiate Activation Lock.

before sell/give away iDevice: What to do before you sell, give away, or trade-in your iPhone or iPad - Apple Support

If you do not know the AppleID and passcode, the iPad is and

will remain useless. Return it to whoever you got it from for assistance

or a refund.

If you were the original purchaser and can prove it with

a receipt from Apple or Apple authorized iPad retailer showing

the serial number of the iPad, make a genius bar

reservation and bring in the receipt, iPad, and government

issued photo ID to Apple and they may assist in resolving the problem.

If the iPad was purchased used or there is no receipt, the AppleID and passcode must be recovered 

It is highly recommended that you make a Genius Bar Reservation to avoid delay at the Apple store

Genius Bar Reservation and Apple Support Options - Apple

Activation Lock page: https://al-support.apple.com/#/getsupport