This phone was bought second hand refurbished a number of years ago. One of my friends broke his phone, the only one for the household, so I decided to try and fix this phone to give to him. The phone was "green-screen" and would not hold a charge at all, obviously requiring a battery replacement.
I took about 30 minutes to repair the iphone using a pair of cheap case tools bought on Amazon, a small screwdriver, and a carpet knife. Everything went as planned after watching a youtube video on the process. Three connectors from the screen to the logic board, 3 connectors from the back of the case to the logic board, and 8 screws holding down the logic board. Removed the old batter probably damaged it beyond use, due to the strength of the glue. Everything went back easily, as long as you are careful with the connectors.
Follow the guides by the letter and don't go to fast. More gentle is better than too rough. All the connectors will "snap" twice when they are in place. Other than that, it was a really simple repair, and a great product from ifixit!