How can I fix the battery

@higenyipeter you can replace a battery but not fix a battery. Since iFixit does not yet have a specific guide for that, use something like this video Slow it down and the music is bit to much a well :-) Start at 5:02 min right up to 7:28 min to see how to remove the battery. Replacement batteries are readily available at many online places.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!