How do I mark a task complete?

When a task is in put in "Client Review" status, a "Mark Complete" button will appear at the top and bottom of that Tracker task's page.  Scroll all the way to the top or the bottom of the task you are looking at and you should see a row of buttons like this one:

Click the "Mark Complete" button and this task will be moved from the "Client Review" status to "Completed" status.

Note: Marking a task complete does not delete that task.  All information contained in that task is still accessible.  Tasks that are marked complete can always be "revived" if needed.