Typically, it is considered best practice to submit/post time accumulated on your weekly time sheet at the end of each day. Once this is posted, the cells grey out.
However, if you work on a current job AFTER submitting/posting all the timesheets for the day, it is possible to add an extra timesheet from within the Weekly time sheet interface.
Weekly time sheet
Once submitted / posted just click on the value displayed for the day in question to open the Quick Timesheet interface:
To add extra time click add and enter the required number of hours and narrative as required. An additional timesheet entry will be created. Click save then OK to return to the main weekly entry interface. The entry for this row / day will now show the total of all timeseets and hovering over this value will display the list of timesheets.
You can use this Quick Timesheet interface to add or subtract hours - if you have entered too much time simply open this window and enter a negative number of hours and asve as above.