I've had my 5s for 32 months, and the battery life seemed to be fading.
I was actually expecting a manual to be packed in the repair kit, but no worries! I found the guide online. It took me about an hour and a half to tear it down and re-assemble. The most time consuming moment was lifting the screen with the suction cup. The suction cup in the kit was not doing the job, luckily I had a more effective one on hand and succeeded with a struggle. The rest of it was smooth sailing and my phone is working properly. However, upon rebooting my phone it said I had 47% remaining on my phone (the percent remaining on my old battery) then it quickly dropped to about 30 (I'm guessing it was re calibrating). I followed the re calibration prompt to drain the battery below 10% (I drained to 5) and then proceeded to charge it in full. I haven't used my phone in the several hours after reaching full charge and it still dropped a significant amount. Not sure if it'll get better over time or not but as of now this battery isn't too much better than my 2.5 year old one I was replacing.
Draw a diagram of your phone on a separate peice of paper and place your parts according to where they go on the real model. Keeps things organized!