You can select and deselect photos in a variety of ways.
Select a single photo: Click the photo or press the arrow keys to quickly navigate to and select the photo.
Select several photos located together: Drag to enclose the photos within the selection rectangle.
Select a contiguous group of photos: Click the first photo, then hold down the Shift key while you click the last photo. You can also hold down Shift and press the arrow keys.
Select multiple photos that are not adjacent to one another: Hold down the Command key as you click each photo.
Select all the photos in a group: Press Command-A or choose Edit > Select All.
Deselect specific photos in a group of selected photos: Hold down the Command key and click the photos you want to deselect.
Deselect all photos: Click an empty space in the window (not on a photo).