Occasionally my iPhone 6S shuts down and indicates that it needs to be plugged in and charged. This happens even though I am sometimes at even 40% battery life. Now I have to worry when my battery gets to this point. I almost never let it go below 10% anymore. I've contacted Apple Support and they ran some remote diagnostics on it and say the battery is ok. They recommend upgrading to 10.1.1 and to see if that helps. I'm currently running 9.3.3 and am weary of going to 10.1.1. Should I do the upgrade and test it or demand that the battery gets replaced? Thank you in advance.