To edit an arc first select it.
Drag either of the arc's end manipulators( 
) to change the start/end angle of the arc.
Drag the arc's diameter manipulators( ) to change the diameter of the arc
Drag any rotate manipulator 
to rotate the arc.
Drag any scale manipulator
to re-size the arc.
Drag the arc to move it.
Arc Properties Dialog
You can also edit the parameters of the arc using its properties panel as shown below. To view the properties panel first select it.Then right-click on it, then select Properties Panel from the context menu that opens.
Click the 
button to show this help topic.
Center
Enter the X and Y coordinates for the center of the arc.
Radius
Enter the X and Y radius for the arc.
Start and End Angle
Enter the start and end angle for the arc in degrees.
Click 
to swap the start and end angles.
Use the Layers drop down to change the layer of the arc.