iPhone 6 Plus will download
My iphone 6 plus they download quickly after subi to version IOS 9.2.1 someone tell me to do please
iPhone 6 Plus, iOS 9.2.1, se descarga rapido
Try to force restart the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. You will not lose any data by doing that, but it can cure some glitches.
If this does not help, setting it up as new device would be the next step:
How to erase your iOS device and then set it up as a new device or restore it from backups
Maybe also take a look at this page about battery drainage