How can I change the radius of the corner of the widget?

To change radius of corners first enter edit mode and click on the pencil icon. Next, click on the style tab. Then, click on the corners tool. Finally select the corners you want and change the radius (length) by adjusting the slider or typing in the number of pixels.

Step 4: Select the corners and adjust the length (C and D) which changes the radius
Note: You can type in the radius pixel size or use the slider to set the pixel size.