I've just looked through the instructions on
iFixit and it's a very difficult procedure. Just to take out the battery took them 111 steps, and it'll be pretty much the same in reverse due to the amount of small connectors and the sheer quantity of glue used to hold everything together.
I would personally just go for the Apple repair service. You could try local repair shops although with them you can't guarantee it will be put back together perfectly.
The possible points that could prevent proper charging is if the power socket, charger, cable, or lightning socket are faulty and not getting a good electrical current through. Otherwise it may be internal (e.g. If heat caused the battery to degrade) and that should only be fixed by Apple. As luckkill4u mentioned, an iPad Air shouldn't develop these problems when it's less than 3 years old (and mine is still going strong after constant daily use), but I can't think of anything else it could be.