I need to update my iPhone

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

NOTE: The iPhone 6 can be updated to iOS 12.5.6, no further.

There are no hardware updates, and no iOS updates for iPhone 6 past iOS 12.

If you require iOS 13 or newer, replace it with a newer or new iPhone.

Preferably replacing it with an iPhone 8 or newer, if you want access to the upcoming iOS 16 version.