My 2011 11" MacBook Air was developing a bulge, and Mac OS X was telling me to replace the battery. Good advice!
Repair went fine. Sucks that I had to get a P5 pentalobe screwdriver. WHY U NO TORX OR PHILLIPS, APPLE?
Repair was way easy. The screws are small but so is the laptop. You'll do fine.
MacBook Air 11" (Mid 2011-Early 2015) Replacement Battery
P5 Pentalobe Screwdriver Retina MacBook Pro and Air