You can reject a candidate from two places:
To reject an applicant from a Job's page...
Find the candidate you wish to reject, and click the Reject button on the right side of their row.
To reject an applicant with a noted reason, click the three-dot button next to the Reject button in the right side of their row.
Select the reason that applies to the candidate, and once it's clicked they will be rejected.
To reject an applicant from within that applicant's Applicant Profile...
Click into their profile and select the Reject at the top of the slider.
To reject the candidate with a connected reason, click into their profile and select the three-dot button next to the Reject button at the top of the slider.
Once you click the reason that applies to the applicant, they will be rejected.
Any time you reject a candidate, with or without a reason, they will immediately be moved the the Rejected stage and their status will show as "Rejected".
If you reject them with a reason, you can review the reason from within their Applicant Profile.