I want 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 12.5.5, no further.

As Stedman suggested, the iPhone 6 cannot be updated past iOS 12. You should consider buying a new phone.