I want to update my phone

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. 

The iPhone 6 can be updated to iOS 12.5.6.

If by update, you want to upgrade it to a newer version of iOS, beyond 12.x, you can't. If you did a simple search here at the ASC, you would have had that answer. Regardless, your iPhone 6 does not have sufficient hardware to be upgraded. If you need to run apps that require a newer version of iOS, you will need to get a newer iPhone.