Double-click the rectangle with the select pointer. A blue handle appears near the rectangle's upper left corner. If several hollow handles appear on the rectangle, this method cannot be used.
Drag the blue handle toward the center of the rectangle.
Release the mouse button when you finish rounding the corners.
You can round rectangles created with Height/Width only if you constrained the rectangle while drawing it.
You can also round rectangle corners by selecting the rectangle and clicking the Point Reshape button on the ribbon.