By dragging the mouse over a track, a grey area shows what you have selected. I would like to know how to “deselect” that area.
If you click on the waveform without dragging, the selected area will no longer be selected.
If you want to delete the selected area, press “Ctrl” and “X” keys , (or “delete” key).
If you want to replace selected area by silence, press “Ctrl” and “L” keys.
More options … http://manual.audacityteam.org/man/edit_menu_remove_special.html
I click on the gray, unused Audacity area below the bottom or last track. That will de-select everything, too.