My brother had his iPhone 6 for about 4 years—almost from the same year it came out—and the battery was just discharging so fast it wouldn't last him half a day. He didn't want to upgrade his phone, but at that time I still wasn't into wanting to repair phones myself. The past few days I watched a lot of iFixit videos and teardowns and just thought to myself, "It would be so awesome to see the inside of an iPhone in person. Just open it up and see it live, how it looks." So I took the plunge and decided to replace the battery myself, and boy was I nervous.
The repair guide was simple, easy to follow, and made the job super simple to accomplish. The only issue I encountered was removing the adhesive strips. One of them ended up breaking off, and I had no idea what to do there after. I kept reading through the guide and found that using a card would help without damaging the battery, so I did and was able to remove the battery with no problem. See, I said the guide was easy to follow and made the job super simple to accomplish. I think it probably took me about 45 minutes to complete the job.
If it's your first job, don't panic. If you follow the guide step by step, you should be able to accomplish the job just fine. I'd make sure to watch the video and read the guide a few times before jumping into the teardown just to make sure you don't miss a step.
Also take your time, if you do this on a day that you have a lot of things to do and are not patient with the repair, you could mess it up.