Hi,
Here’s a [https://www.dfrsolutions.com/blog/why-do-lithium-ion-batteries-swell|link] that describes most of the reasons that can cause a swollen Lithium battery.
The battery will have to be replaced and responsibly disposed of as it is dangerous in this condition.
Here’s the Ifixit [guide|121285] guide that will help.
Ifixit also sells replacement batteries for the phone. Just click on the ''Parts'' link in the guide.
If doing this yourself is too daunting, contact a reputable, professional mobile phone repair service and ask for a quote.