How can I update my iPhone to iOS 12

You will probably need to use iTunes on your computer to accomplish the update, as you are several versions of iOS behind. Do note that any update will be to iOS 15.4. 

Update your iPhone, iPad, or iPod touch with iTunes - Apple Support (CA)

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support (CA)

It would depend on the version of iPhone you have. However, given that you are asking about updating to iOS 12, my guess is you have a much older iPhone that is stuck on a version of iOS that is older than iOS 12. My guess you have an iPhone 5, 5c or lower, which none will be able to be updated past iOS 10.3.4. For a newer iOS, you would need to purchase a newer device.

And what version of iOS is on the iPhone?

Hello ~ In that case take a look here:

If your iPhone, iPad, or iPod touch won't update - Apple Support

~Katana-San~