First step is to remove the battery before it permanently damages the laptop. This Dell support page has more information. Next call Dell tech support for more information but if the unit is out of warranty it is possible that no repairs will be forthcoming. In that case, and assuming no further damage has been done to the laptop, batteries are available for purchase here. I would suggest that in the future if you notice swelling, no matter how slight, you address the problem right away.
Hi! Noticed the touchpad raised a little bit up since the end of 2020 way before the warranty expired. I never use touchpad and did not pay attention to the little deform. Recently it raised way up there and checked online and learned it is the battery issue and dangerous. I wonder whether Dell can provide an out of warranty replacement of the failed battery? Or what options to fix the issue? Thanks. 
Battery swollen, Touchpad raised up, out of warranty