Before the resetting of Android phones, you'd better transfer important files from Android to computer as a backup. You can follow the steps below to factory reset your Android phones.
Step 1. You can pull down the notification shade.
Step 2. Then you need to tap on the icon on the top right corner.
Step 3. Just click on the gear icon to click on the Settings.
Step 4. Select the Backup & Reset button.
Step 5. Click on the Factory Data Reset.
Step 6. You will receive a notification that remind you all the files will be erased and you can just confirm it.
Step 7. The start screen will show up if you finish the reset.
Step 8. Then you may be required to enter the account or other information. Just click on the Not Now button to skip to the next step.
Step 9. Finally just open the app tray and start the Gallery to see if there is still photos in it. If there is no photo in the device, that means your device has been reset successfully.