Just to elaborate on Harry's response, the simple answer is that you can't fix a swelling battery; your only option is to replace it.
The swelling is caused by gasses building up inside the sealed plastic layer, and those gasses are caused by the chemical reactions inside the battery breaking down and no longer working correctly. So the very act of swelling right there tells you the battery is failing, and the swelling can easily cause problems like dislodging the screen or back of the phone. If it goes on too long, eventually it's possible for the battery to catch fire (remember the Galaxy Note 7?), so you really do want to replace that battery ASAP.