Just delete the dimension (pick it and hit delete on the keyboard), click the dimension icon, pick the circle with the left mouse button once, and locate the dimension with the middle mouse button to create the radius dimension.
Highlight the diameter dimension, then click Edit > Replace. Then create a radius dimension. The advantage of using replace is that the dimension will automatically be replaced on the drawing. If you delete it, then make a new one, the dimension is lost on the drawing and you will have to show it again.