Your iPad Air battery will have lost some capacity over its life - and will not be able to provide the same battery endurance as when it was new. Like us, the battery ages and becomes less effective over time... but by how much will be largely dependent upon how heavily it has been used - and cared for over its life.
Apple Stores do not offer a battery replacement service per-se, instead, for qualifying iPads, offer an exchange programme where you exchange your current iPad for a reconditioned device of the same model and specification. The replacement cost, assuming that the iPad is otherwise fault-free and in good condition, is understood to be in the region of £99. If other damage is evident, the out-of-warranty exchange is often quoted as c.60% of the “as new” price of your current iPad.
If an exchange is offered, you’ll need to weigh the relative age and performance of your current iPad against this cost. Given that the original iPad Air cannot be updated beyond iOS 12.4.7, some Apps are no longer available/compatible with your iPad; as Apps cease to be compatible as they are progressively updated, or as older versions of Apps are withdrawn from the App Store, older iPads that are beyond update support will become increasingly limited in their usefulness and utility.
If you instead opt to use an Apple Authorised Repair Agent/Provider, your options may be different in that a battery replacement service might be available, however, the costs may differ. You must still consider the relative merit of extending the life of an iPad with diminishing utility.
I hope this information and insight proves to be helpful to you.