You can change the candidates assigned to a requisition at any time, regardless of the requisition status.
To edit them, simply click on Modify next to the associated candidates in the requisition history (on the right side).
You can delete a candidate by clicking the "x" button next to their name.
Attention:
If a requisition is completed or closed, you can only replace the candidate(s) associated with that requisition. You will not be able to remove a candidate without replacing them with another.