To Modify the Sweep Angle of an Arc

With AccuDraw active, select the Modify Element tool.
Select the arc.
From the Method option menu in the tool settings window, choose Angle.
The drawing plane origin moves to the center of the arc, and the coordinate system aligns its x-axis with the start of the arc's sweep.
(Optional) Rotate the drawing plane axes.
(Optional) Preview the modifications by keying in desired changes in the AccuDraw window.
Enter a data point to complete the modification.

Modifying the sweep angle of an arc

Note: You can adjust the arc sweep by either complement of the angle (180° either way).