If you're using the app: Go to My Jobs, then tap on Upcoming jobs. Your job should be listed here. Find the job, select it, and then hit Unapply at the bottom of the job window.
To un-apply on the Web Portal: Click on the Applied Jobs tab and then click on the appropriate job post. This opens a job details window. Just click Unapply at the bottom of the window to withdraw your application.
Un-applying for a job will not affect your cancellation score on your profile. However, if you cancel a confirmed job, your cancellation score will be impacted.