My wife and I were both having battery issues with our Iphone 6's. It got to the point of a 3-minute youtube video draining our phones from 100% to 1%. My phone couldn't be away from a charging point for more then 2-3 minutes. Not a good thing when you need to be available at a moment's notice. Upon examining my phone with ibackupbot I found that my battery had been cycled over 900 times and was able to hold a max charge of 100 mAh (yikes)!
Ordered two replacement batteries with kits. First up was my phone. Thought it would be best to possibly kill my phone and not the wife's. I followed the directions off the iFixit web site to the letter. Removed all of the connections to remove the screen from the phone. Had a bit of an issue with getting the digitizer to remain connected during reassembly, but got it to stay in place on second attempt. Phone powered right up. On my wife's phone I decided to leave the screen attached during the process. Was rather nervous about this at first, but was able to firmly clamp the screen in a 90-degree position. That enabled me to replace the battery in under five minutes. 15 minutes for my phone, 5 minutes for the wife's phone.
A large magnifier with a light was extremely helpful during reassembly. Particularly with the 5 connections to get the screen reattached. The iSclack was a phone saver. With the delicate attachment cables at the top of the phone, it ensured that I didn't rip the cable in half when opening the phone.