Click on the shift you wish to delete. In the opening window, in the bottom, click on the garbage-can icon. Click again on the same icon to delete the shift.
In the app, click on the shift you want to delete. In the bottom you can click on the delete button.
You can also delete multiple shifts with the bulk-delete option. Click on the menu button on the top right with the three dots, and then on bulk delete. Now you can filter the shifts you wish to delete and then click on delete shifts on the bottom right of the window.