I want to update my iPhone

To update to the maximum of iOS 12.5.7, please 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. 

Tap Settings

Tap General

Tap Software Update

If an update is available for your phone, it will let you know