You can delete an Employee Shift after you have assigned it to an employee. Go to Employees > Shifts.

You can either delete the shifts one at a time, or simultaneously. To delete one at a time, tick the box of the Shift you would like to delete and click the Trash button next to the Add Shift button.

A warning prompt will appear.

Click Yes to proceed with deletion, click No to cancel. You will be led back to the page and the Employee Shift you just deleted is no longer visible.  

If you want to delete Employee Shifts simultaneously, click the tick box at the first column and a Trash button will appear next to the Add Shift button.

Click the tick boxes of the shifts you want to delete, then click the Trash button.

A warning prompt will appear.

Click Yes to delete the shifts. Otherwise, click No. You are ready now to assign new shifts.