Portrait mode
My iphone 6 is locked in portrait mode. I'd like to unlock.
iPhone 6s, iOS 12
To change mode, swipe left or right at the bottom of the screen in the Camera app. See: Use Portrait mode on your iPhone - Apple Support
or if you mean rotation lock, open Control Center by swiping up on an iPhone 6s starting from below the home button and then tap on the Rotation Lock control. See: Use and customize Control Center on your iPhone, iPad, and iPod touch - Apple Support