To delete a single shift, just tap the shift you'd like to delete on your Calendar screen.
On the Shift Details screen that appears, simply press the red "Delete Shift" button and the shift will be deleted. Your calendar will automatically remove the shift and it won't appear on your schedule anymore.
To delete multiple shifts at once tap the gear icon on the Calendar screen.
Then, in the Change Multiple Shifts at Once section, tap Delete Multiple Events. Select the shifts you'd like to delete. Once selected, press "Delete" in the top-right corner. A second notification will appear to confirm you want to delete these shifts. Confirm and your calendar will update accordingly.