Google- "where is iOS backup stored on Windows" to find the backup. Make a copy before updating to iOS 9 in case a new backup overwrites the 8.4 backup. Then u are good to go.
I don't run iTunes on a PC any more, but if it's similar to the Mac version, in Preferences go to Devices then right mouse the relevant backup and pick Archive from the menu.
I don't run iTunes on a PC any more, but if it's similar to the Mac version, in Preferences go to Devices then right mouse the relevant backup and pick Archive from the menu.