To delete multiple files at once, select the files and click on the delete icon in the lower right corner.
To select multiple files at once, press Shift, click the left mouse button and draw a blue selection frame anywhere in edit mode. Any file that is inside the frame will automatically go into your set. The files in the set can then be deleted with one click.
If you just want to delete one single file you can use the three dot menu on the chosen file and click “Delete”.