On the Roster View, locate the candidate you want to remove from the assigment and click on the block.
On the pop-up screen, click on the Cancel Assignment option.
Select whether you want to notify the client and the candidate and then click Re-Assign.
This will take you to the job matching window where you can assign a new candidate.
You can also use the Click to find candidates option on the Roster View against the assignment to assign another candidate.