Change the corner radius of a rounded rectangle

The corner radius determines the roundness of the rectangle’s corners.

To change the default corner radius, choose Edit> Preferences > General (Windows) or Illustrator > Preferences > General (Mac OS), and enter a new value for Corner Radius. Alternatively, select the Rounded Rectangle tool, click in the document window, and enter a new value for Corner Radius. The default radius applies only to new rounded rectangles you draw, not to existing rounded rectangles.
To change the corner radius while dragging with the Rounded Rectangle tool, press the Up Arrow key or Down Arrow key. When the corners are the desired roundness, release the key.
To create square corners while dragging with the Rounded Rectangle tool, press the Left Arrow key.
To create the most rounded corners while dragging with the Rounded Rectangle tool, press the Right Arrow key.

Illustrator