How do you change the URL alias of a contact form?
I went under Configuration --> URL Aliases, and added a new alias for /contact-us that points to /contact/contact_us

I expected the result to be that anyone typing in /contact-us would end up at /contact/contact_us, but instead the opposite is happening. Anyone typing in /contact/contact_us is now redirected to /contact-us and the page is shown there.

This is a little different from how Drupal 7 works, but this is the answer I needed.