The first thing you'll want to do is go to the Applicant view:
Then, click on the applicant's name to open up his/her profile. If you see that the background check status is "Pending," it means the check has not yet been completed. If you want to move this applicant to Rejected, click 'Actions' and then click on 'Move to Stage...'. From here, select 'Rejected' and the applicant will be moved to the Rejected stage:
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