If you've applied for a shift you no longer want to work, and your application is still live, then you'll need to withdraw your application.
To withdraw your application, head to 'Applied for' in the app side bar, tap onto the shift, and tap 'withdraw' in the grey action bar at the top of the screen.
If possible, it's always better to withdraw your application rather than cancelling a shift after you've been hired, as it will have less of an impact on the hirer, and you won't risk a late cancellation. For more information about late cancellations, take look at this article.