My iPhone 5 is around 4 years old. The battery wasn't holding a charge well anymore.
I referenced a YouTube video showing the replacement. It claimed it could be done in 2 minutes, which is probably right if you do a lot of them. It probably took me around 5 - 10 minutes. I didn't time myself. The iFixIt kit had everything that I needed to get the job done, including the proper screwdrivers for the tiny specialty screws. The screwdrivers are magnetic, which helped keep from losing the tiny screws.
I did the work on a large cookie sheet with 1/2" sides so that if I dropped anything, it would be easy to find. I made a mistake in removing one wrong screw and ended up removing a piece that didn't have to be removed, but that was a minor challenge. There was also one tiny screw that was slightly longer than the other tiny screw and I got that one in the wrong place initially when I was putting it back together, but it wasn't difficult to figure out the problem. The new iPhone 5 replacement battery is working much better than my old battery and will hopefully let me delay buying a new phone for another couple of years. Be sure to recycle your old battery as e-waste. Don't put it in the trash. My husband said it was pretty cool to watch me sit down and replace my own phone battery. We got together when I had him over to help me open up a computer and fix it because I was afraid to do it myself 20 years ago. I'm not afraid of electronics anymore.