I can't upgrade to iOS 12

No place to download what, the update? Once you have connected your iPhone to iTunes, you must select the iPhone in the sidebar, and then select Update after the contents of the iPhone appear. From there, it will check and update your device.

Hit the button that says "Check for Update":

Follow the steps here: Update your iPhone, iPad, or iPod touch - Apple Support

https://www.apple.com/itunes/download/win64

What device are you trying to update? Settings->General->Model

What happens when you try, exactly?

If you have an iPhone 6 then yes to can update to iOS 12. Try to update with iTunes if OTA doesn't work.