Steps To Downgrade iOS 9 to iOS 8.4 :-

1. First of all launch iTunes app in your PC.
2. Now connect your ios device to your computer via USB cable and wait for itunes to detect your device.
3. Now a popup will appear with Restore option.
4. Now click on Restore and Update option there.
5. Now itunes will automatically detect the latest version that is ios 8.4 and then click on next button on the box to update software of your ios device.
6. Now a terms and condition box will appears and you have to agree that and downgrading process will start and it can takes time to complete, so be patient with this.
7. After sometime process will completed successfully and your ios device will get downgraded and now you can easily enjoy the ios 8.4 and can get ios 9 official update on your device.