Since you most likely don't want your iPhone 6 to die a sudden death, you should educate yourself on "Error 53."
The Guardian recently reported on "Error 53," an issue with iOS 9 apparently affecting "thousands." Once installed, the new software may "disable" your iPhone 6 if it detects that your iPhone's home button has been repaired by a third-party technician, meaning, anyone not with Apple. This is detected though your iPhone's Touch ID system. However, The Guardian also reports that individuals with damaged phones that have not been repaired have experienced the so-called "Error 53."
Apparently, the error only shows itself after the iOS 9 software has been installed. So, the main issue here is that iPhone 6 users might have been unaware of the danger, installed the software after a non-Apple repair, and subsequently had their iPhones "bricked," in other words, "made useless." That doesn't sound like fun. But, Apple just responded to The Guardian's story, claiming that it is aware of "Error 53" and that the error protects the security of iPhone users. The unnamed Apple spokesperson was quoted as follows: