My battery started bulging back in 2010. I hadn't realized that it was actually a fire hazard, since, well, nobody thought anything of it. Luckily, I found out that it could actually turn my apartment into a literal hellhole (as opposed to the figurative hellhole it currently serves as today), found iFixit and got that new battery. Now I can be safe from the Lithium overlords. For now...
Could not have been easier. Take out - replace.
Replace your battery ASAP if it looks like that fat guy from Monty Python's The Meaning of Life. It might actually just a-splode.