To adjust the Billing or Payroll amount, or the duration of a completed Task, simply right click on the Task in question. Then select the Reconfirm Billing/Wages option highlighted:
Once you have clicked Reconfirm Billing/Wages, the Task will revert to a status of Not Confirmed. You can then choose to Confirm it in a number of ways, just as if you were confirming it for the first time (see below).