Bought a used iPhone 6 from Craigslist as a replacement phone. It was in great physical condition, but when I was forced to do the iOS update, the phone grinded to a near-halt. It took seconds to change screens, opening apps crashed, typing was painful as the keyboard was 20 keystrokes behind me all the time, etc. You know the problems—this phone had them all.
Honestly, the scariest part of the whole project was ordering the battery. I'd read and heard all the "no user-serviceable parts inside" stories, and I was worried this was a non-starter project. Once I read all the encouragement online, read the guide multiple times, watched the YouTube videos, etc., I figured that if everyone else can do it, I can too.
It really wasn't hard, and getting the tool kit along with the battery made it a walk in the park. I just took my time, laid everything out carefully, followed the instructions step-by-step and it was finished before I knew it. Even the glue removal was easy, once I'd watched the videos on how to pull it out and knew what to expect.
Nope, just take your time and follow the instructions. I'm admittedly pretty mechanical, but electronics do worry me a bit. This really couldn't have been a lot easier to do.