You can resort to iTunes to restore iPhone to factory settings. Just follow below steps to do that.
Restoring iPhone to Factory Settings with iTunes
iTunes enables users to back up data to iTunes and restore iPhone to factory settings. Before you start restoring iPhone to factory settings, please note that you need to install the latest version of iTunes on your PC or MAC. Then go to Settings > iCloud to turn off Find My iPhone. Now, the detailed restoring steps are as follows.
1. Plug iPhone into the computer with the USB cable. The iTunes on your computer will automatically launch. If not, double-click the icon to start it.
2. Click the name of your iPhone at the upper right corner of iTunes, and then the information of your iPhone will be shown. Click the "Restore iPhone" button in the window to start.
3. iTunes will ask whether you want to back up iPhone before restoring. You'd better choose "Yes" for the reset will erase the contents in your iPhone. Then iTunes will start restoring your iPhone to factory settings.
3. Click Restore again to confirm. Then iTunes erases your iPhone and installs the latest iOS or iPod software. After you restore your iPhone to factory settings, it will restart. Now you can set it up as new.
For more methods, check:
How Do I Restore My iPhone to Factory Settings
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