How to update an ios 12

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.

If you device is older and running iOS 12.5.5 it’s probably updated as far as it’s hardware will allow.

What model device are you using?