Ha.ha. I know. However since iOS 8.4 gave me really good impression about new music app, while iOS 9 provided so many impressive new functions, I decided to update, and I didn't know iPhone software could not be downgraded through update. I thought this was possible because I downgraded iPad mini software from iOS 9 to iOS 8.4.
OK. I would better charge my phone as long as I have power source.
But you CAN downgrade your iPhone via DFU. You just may not be able to restore your backup.