Once you have a selection, you can adjust its boundaries, e.g. after zooming to get more precision
Press [Shift] and left-click at a new position. The nearest boundary will move to it when the [Shift]-key is released.
Press [Shift] and hold the left mouse button down, and you can pull the boundary to and fro until it is in the right place. Be careful, if you forget to press [Shift], you will simply drag a completely new selection.
A dialog box opens that lets you enter the precise times of the beginning and end of the intended selection:
The dialog opens with the current cursor position times
Replace them with your own intended selection boundary times
There are other useful adjustments in the Select menu, such as moving boundaries to the nearest zero-crossing in the signal. This is useful for avoiding spurious clicking sounds when playing back selected segments (clicks are usually due to transient energy debris isolated when the sound wave is left in mid-air at the selection boundaries).
A segment is deselected when the cursor is clicked elsewhere in the waveform
Alternatively, if you do not want the cursor to be visible, click just outside the waveform to the left or right. Or, open the Select menu and choose Move cursor to; then enter a time point that is beyond the currently visible waveform in the Sound editor.