How to action the timesheet when the contractor submitted the timesheet for incorrect number of days and it was approved by the manager. The timesheet has not been online validated by the agency

When you come to validating a timesheet you have the option to 'Reject' the timesheet. This option can be used when the timesheet needs amending. When rejecting a timesheet you must write a reject note, for example 'incorrect number of days'

Another way you can do this without rejecting the timesheet is to just reopen the timesheet from timesheet display.