phone dead, will not charge, recently updated to 9.2

Hi, I recently bought my wife an apple watch last holidays. In order to use the watch, she needed to update her phone (iphone 6 128gb) to the latest OS. After 2 days updating , she noticed her battery draining faster than usual. One day it got drained completely and will not charge at all. Options tried: charged with different USB charger, wall sockets and mac book, overnight duration; had apple genius appointment and said only "fix" was to swap it which means simply buy a new one (it is out of warranty); had a local repair store use different battery and charging ports but still won't respond. We Any idea/ suggestion highly appreciated! thank you!