How do I update iOS on my iPhone 6

Not beyond iOS 12.5.6. See why in this compatibility list:

These are the instructions to get 12.5.6 version:--> Update your iPhone, iPad, or iPod touch – Apple Support

Regards

An iPhone 6 isn't capable of 13 or above. To install 12.5.6, use a computer or the Software Update function in Settings.

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On your iPhone go to Settings > General > About > Version and tell us what it there.