It's quite easy. You can reach this aim through Settings app. Just follow below steps one by one.
You will also be able to reset iPhone to factory settings within your iPhone. You just need to go to "Settings > General > Reset" and then choose the categories you want to reset and then apply it. If you choose "Reset All Contents and Settings", then you will get all the files in your iPhone erased.
Once you tap on "Erase All Content and Settings", you will be asked to enter your Apple ID password. Next, you will see a Popup Message informing you that All media, data and settings will be erased from your iPhone. Click on Erase iPhone to factory reset iPhone 6 Plus.
For more methods, check:
How to Restore iPhone 6 Plus to Factory Settings
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