Shortcut: enter or double-click
Since every shape consists of multiple vector points, these are all editable. After you have selected the shape simply press enter or double-click on it to edit the points.
The vector points are indicated by little circles instead of the little squares of the layer handles. To go back to editing the whole shape, press esc or click anywhere on the canvas.
If you want to know more about the different modes that are available for vector points read the tip Vector point options.