I have an ipad Air 2 and it keeps losing battery, what can I do to stop this happening?

If the battery has reached the end of its serviceable life, other than using your iPad whilst connected to its Power Adapter, there is nothing that you can technically do.

If you take your iPad Air 2 to the Genius Bar at your local Apple Store (when they reopen), as Apple do not offer a battery replacement service for iPad, you might be offered an our-of-warranty exchange for your existing iPad.

The typically reported cost, for an out-of-warranty exchange, is c.60% of the “as new” cost of the iPad - although if the exchange is just for a battery and the iPad is otherwise in pristine condition, they cost may be lower.

They do offer battery repair or device replacement for a lower battery service fee according to the link I posted above.

https://support.apple.com/ipad/repair/service#battery

If your battery needs replacement, we can repair or replace your iPad for a battery service fee.

To clarify, in many regions, Apple do not offer a service to replace the battery within a customer’s iPad.

However, for qualifying devices and in lieu of an individual battery replacement - Apple do offer an iPad exchange programme, for a reconditioned iPad of the same model and specification, for the cost of an iPad battery replacement.

An AASP may handle battery service differently.

Replace the battery with a new one. At over 6 years old its entirely possible the battery has begun to fail.

iPad Repair - Battery and Power - Official Apple Support

The iPad Air 2 is quite old, from late 2014. You may need your battery replaced. Visit an Apple Store for that, but make an appointment first (online).