Once a timesheet is submitted, it cannot be modified by the user. This article explains how to modify a timesheet after it has been submitted.
1st case: the timesheet has not been approved yet
The approver can deny the timesheet approval. The timesheet will then go back to temporary status and the user will be able to modify it before submitting it again.
2d case: the timesheet has been approved
The administrator can invalidate the timesheet by going in Submit > Select the user > Select the timesheet > Click on "Invalidate" button in the top right corner:
An approver can also do this if his/her role gives the permission to invalidate timesheets belonging to one's collaborators.