Depending on your role, you have different permissions for timesheet deletion as follows:

For agency admins: You can delete a timesheet in any status (Pending, accepted, rejected) at any time.

For contractors: You can delete your timesheets only whilst they are in the pending status.

Authorisers: You cannot delete timesheets.

Go to wherever the timesheet is, this could be the timesheet summary page, under the contract detail (timesheets tab) or within the contractor detail page (timesheets tab).

Hover over the timesheet and under the 'Actions' column to the right, click on the bin. Give your reason when prompted and the timesheet will be deleted. 

You will now be able to submit a new timesheet for that contract and that date if required.