Mhd Znd, Download 3uTools and connect your phone to your computer (Windows as 3uTools is for Windows).
When connected and authorised, you can enter the Flash menu. Download the latest iOS package in 3uTools (it will list out the compatible, signed, versions). There is an option there in the Flash menu, that is made to prevent the recovery of data. Select that option before clicking the Flash button!
***NOTE***! I haven’t done a recovery test on a device after using that option, so I can’t attest to it’s reliability personally!