iPhone 6S Plus battery was dying on me pretty quickly (extreme overuse). It was just its time to go.
Watched the repair video and followed the step-by-step instructions. Pretty straightforward. Took longer then I expected but then again it was my first time taking this phone apart.
Popping the screen off takes a little more elbow grease and finesse than the video showed but I'm sure that's on a phone-to-phone basis.
Some good advice is to be extremely extra-diligent when it comes to removing the adhesive strip from under the battery. I removed one fairly decently according to the instructions but screwed up and lost a a couple under the battery when it tore (if you've done this you know what I'm talking about) . I didn't have the heat option so I ended up using some dental floss to sort of "saw" away underneath the old battery. Be very careful with the old battery. I also recommend either doubling up the dental floss (it kept breaking on me) or using something stronger like fishing line (and be careful not to cut your fingers on the string).