UPDATE
I ended up calling UA Web Support (via the reservations phone number). Basically, since Trip Insurance is a separate charge to your method of payment you need to contact the Trip Insurance provider. In my case, I had to call Allianz Global Assistance.
I had to wait until AFTER I got the email with the insurance policy number from Allianz. The policy number is in a PDF attachment to the email. The email is sent from
[email protected]
I called 800-496-6821 and selected Option 4. The agent canceled my policy and sent me an email confirmation.
The trip insurance cancellation policy is:
We will refund your insurance premium if you cancel your insurance
within 10 days of purchase and have not filed a claim or departed on
your trip.