You can easily reposition a marquee as you draw it by pressing the Spacebar while drawing.

To reposition a selection with the Spacebar:

  1. Begin dragging to draw the selection.
  2. Without releasing the mouse button, hold down the Spacebar.
  3. Drag the marquee to another location on the canvas.
  4. While still holding down the mouse button, release the Spacebar.
  5. Continue dragging to draw the selection.