Cancelling a signup can be done on a mobile device or on a computer.
In the app, your opportunities can be viewed on the main page. Select the opportunity and choose "Cancel Reservation" at the bottom of the page.
If you'd like to cancel a signup on a computer, make sure you are logged into your Golden account, and select your name in the top right-hand corner. Select "my opportunities" and then select "cancel" for the specific opportunity you would like to unregister for.