How can I 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.

iPhone 6 is an outdated iPhone it can’t be update to a higher iOS (13 and later).

The last security updates iOS 12.5.7 -> released September 12, 2022, available for iPhone 6.

Go to Settings -> General -> Software Update -> Download and Install.

Yes, most apps require iOS 13 or later.                                

Considering a newer type of iPhone?

Read this before you buy a newer iPhone.

Apple Store -> iPhone - Buying iPhone - Apple

Apple Certified Refurbished -> Refurbished iPhone Deals - Apple