Round the corners of a rectangle
Select the rectangle that you want to edit.
Do one of the following:
On the artboard, move your pointer over either end of the dotted lines that jut out from the top left corner of the rectangle, and then drag when your pointer appears as a corner radius handle .
By holding down SHIFT while you drag either corner radius handle, you can edit the X and Y corner radii individually.
Under Appearance in the Properties panel, change the values of the RadiusX and RadiusY properties.