Where is the cancel button when editing a photo?

You can revert any adjustment individually, by clicking the blue checkmark to the right of the adjustment.

What I am doing, before experimenting with new edits, is to duplicate the image version with the current adjustments (Image > Duplicate Image). Then I am editing the copy. If I do not like it, I simply delete the copy and are back to the previous version. The duplicate does not need much extra storage, because it is sharing the original with the other versions of the same image.

Hi - Agreed - that would be nice

You can cmd-z (undo) though - repeatedly until you get back to same state as when you started editing. Then click done.

At the bottom of the "adjust" pane, at least, there is a "Reset Adjustments". Otherwise, undo.