To remove a photo from a stack, begin by expanding the stack. Then Control-click (PC: right-click) directly on that photo to display the contextual menu shown in Figure 8. Choose Stacking > Remove from Stack. This action doesn’t delete the photo, remove it from a collection, etc. Lightroom just takes that photo out of the stack. If I removed just one photo from the stack in this example, when I collapsed the stack again I’d have two thumbnails in the grid—one representing the 11 photos still stacked, and a second thumbnail of just that individual photo I removed.
If you actually want to delete a photo in the stack (not just remove it from the stack), expand the stack, click the photo, and press Delete (PC: Backspace) on your keyboard.