If you haven't been hired for the job yet, you can withdraw your application via the My Applications page.

If you have been offered the job, you'll need to make contact with the employer to let them know that you can't make it. 

Remember, it's very unprofessional, and against our Rules of Conduct to not show up for a job that you've already committed to. By not adhering to our Rules of Conduct your Student Job Search account may be suspended.