I wanted to put a new battery in my aging iPhone 6 to restore some performance.
The original battery was more difficult to remove than I expected, even after reading others' experiences. The pull tabs on the original adhesive both broke well before I got much out from under the battery, even though I tried to pull it as demonstrated in the videos. It took a lot of prying and wrestling with the battery to get it out of the case after that. I removed the display assembly before removing the battery and am very glad I did, since it took so much work to get the old battery out. It was difficult to get the replacement battery's cable seated and aligned properly, and it was therefore hard to connect it back to the logic board. I was able to complete the repair successfully, but it was a lot harder and more stressful than I anticipated.
Be prepared for the worst when it comes to getting the old battery out and remove the display assembly first. Make sure the phone will boot up before you completely reassemble it.