You can unapprove time by using the "Summary" tab on Timesheets. You'll need to select the employee by clicking the Options button to bring up the options panel on the left hand side of the screen, then select the employee from the drop-down list.
The employee's time for the week will be displayed. If the time has been approved, then the Revoke button on the top right of the screen will be shown. Click Revoke to unapprove the employee's time for the week.