Solution : Restore an iPhone in iTunes
Just as advised in the way mentioned in the above, you also need to backup important files saved on your iPhone first. Then:
> Connect your iPhone to the computer and launch iTunes on the computer. Select your iPhone from the row of buttons along the top of the window. Select This computer and then click Back Up Now to create a backup files in iTunes in case of data losses when you finish restoring your iPhone.
> Now, select your iPhone in iTunes and click Restore iPhone option. Then you will see a pop-up window which confirms again whether you want to erase everything on your iPhone or not. Then just wait for the process to complete and then your iPhone will have been successfully factory reset in 15-20 minutes.