There are two ways to rotate a stamp:
Select the stamp that you wish to rotate. Then click and drag the handle to rotate the stamp.
Note: Hold the Shift key while dragging to rotate the stamp in set increments.
To access the Rubber Stamp properties, right click (Mac: Ctrl + Click) on the annotation and choose Properties in the mouse menu or click on the edit button on the Mini Toolbar. Then change the rotation value to the desired rotation