How do I factory reset my phone

See these support articles:

Restore your iPhone, iPad, or iPod to factory settings [with a computer] - "A factory restore erases the information and settings on your iPhone, iPad, or iPod and installs the latest version of iOS or iPod software."

Click here --> http://support.apple.com/HT201252

How to erase your iPhone, iPad, or iPod touch

"Learn how to wipe information from your iPhone, iPad, or iPod touch and restore your device to factory settings." Without using a computer. Does not reinstall iOS.

Click here --> https://support.apple.com/HT201274

If neither of the above work, read the "If you can't erase your device" section in the last link, including the last part about contacting Apple.

This link will lead you to an explanatory video of apple support about your question.

https://youtu.be/g6tppaNiEHY