Apple and ifixit have complete guides for take apart and replacement on their websites.

My recommendation is to buy a real Apple battery if you’re going to do this. Lithium ion batteries from shady sources are a good way to ruin your phone or worse, start a fire.

Apple actually offers a flat rate battery replacement for recent iPhones and in my opinion it is worth the money.

https://support.apple.com/iphone/repair/battery-replacement