We know that the shortlisting process is an iterative one, and the shortlist may change as new candidates complete their interviews.
In shortlisting view, if you have a candidate in the shortlist but change your mind, simply drag them back over to the main list on the left.
At any time, you can change the status of a candidate (although bear in mind the implications if you have already sent them a rejection email for example). Simply update the status in their review page. Remember, that if you have already rejected them, or have them in a custom section of your workflow, you may need to set the correct options in your filter to see them, as they may be hidden by default.