How to select and move the shapes in Preview
Use the keyboard shortcut Command + I to show the Inspector. It will list all the shapes added in the current editing session.
Click on the desired shape in the Inspector. This will also select the shape.





Now use the arrow  keys to move the selected shape, Up ↑, Down ↓, Left ← or Right →.
Press and hold Shift key along with the arrow key to move the shape with faster speed.
Alternatively, once the shape is selected (either by clicking on it or via Inspector), you can also hover the mouse pointer over it until the cursor turns into a white glove. Thereafter, you can click and drag to move the shape freely.