swollen ipad battery...needs replaced?

bought an ipad mini 2 off ebay…came with a swollen battery (go figure)

I paid about $60 for it, the seller offered a partial refund of $40 because he forgot to mention it…so would end up being $20 (a steal)

he said he never had a problem with it. i’ve been using it for two weeks now and never had a issue with it. screen works fine. charges and holds a charge just like new. so my questions are

(1) is it dangerous? like does it require replacement or am I OK to keep using it as is?

(2) can I, by myself, replace the battery? I’ve replaced two iphone batteries, it’s pretty easy but those batteries weren’t swollen. Or is a swallow battery some risk factor and only an expert can replace it?