How do I update iPhone XR from iOS 16 to iOS 17 instead of iOS 18?

You either update your iPhone XR to iOS 18 – or force it to remain on iOS 16.

There was a time right after Apple first released iOS 18 when you could choose whether to update to iOS 17.7 or to iOS 18.0. That time has passed, and Apple no longer offers the option of updating only as far as iOS 17.

That's correct. iOS 17 hasn't been signed for some time. You can ONLY update your phone to the most current public release of iOS, which is currently iOS 18.3.1.

You cannot update to iOS 17.