You can only make changes to timesheets in the current, open pay period before payroll closes and processing the timesheet.  See the appropriate user guide for your policy profile for instructions on editing time on your timesheet. 

After a timesheet has been processed, any changes must go through the amended timesheet procedure.  Timesheets can be amended up to 365 days in the past.  You will only amend your timesheet to correct misreported time or leave.

See the Employee Amended Timesheet FAQ or Supervisor Amended Timesheet FAQ for instructions on requesting changes to past timesheets.