It is important to note that upgrading from your current version of iOS is only possible whilst Apple is still signing the older version of iOS firmware, once that signing window is closed then downgrading will not be possible. You can check which versions are being signed from here

Before any attempts of downgrading are made make sure you backup your data like Photos and stuff to iCloud or locally to your PC before proceeding, note that full Backup option available in iTunes and iCloud won’t work if you try to restore a backup made from a newer firmware onto an older version. 

Step 1 

Download the current signed version of iOS 9.0.2 firmware file for your device. 

Step 2:

Start iTunes, head to the device’s 

Step 3:

Now hold down the padding: Option / Alt key on your Mac’s keyboard, or Shift key if you are on Windows, and click the Restore iPhone button.

Step 4:

You will now be prompted to browse and select the IOS ipw file you downloaded earlier

Step 5:

Select and open the file, which should begin the downgrading / restoring to iOS process.