You can use the following methods to delete the lowest level footage in Final Cut Pro:

Select the lowest-level material to delete: find the lowest-level material on the timeline and click Select. You can click on the material with the mouse or use the arrow keys on the keyboard to locate and select.

Delete the lowest-level material: press the "Delete" key on the keyboard, or right-click the material and select the "Delete" option to delete the lowest-level material.

Please note that if the bottom material depends on other materials, deleting the bottom material may affect the display of other materials. Before deleting, it is suggested to check whether other materials depend on the lowest-level materials, so as not to affect the production of the whole project by mistake.