How to factory reset my iPhone 6 without using itunes?

from Restore your iPhone, iPad, or iPod to factory settings - Apple Support

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... you can erase and restore your device without a computer.

How to erase your iPhone, iPad, or iPod touch - Apple Support

You can "erase all content and settings" without iTunes, but this is not considered a reset.

A reset requires iTunes because it will download and install a new fresh copy of the the current iOS, where erase all content and settings does not.

What do you mean by hard reset? What are you attempting to do?