Adjusting with the on-screen handle
You can set the border radius by dragging the anchor point (adjustment handle) inside the box selection frame.
Setting it from the Box tab in the right panel
Select the box whose border radius you want to change. To select multiple boxes, hold Shift and click each box.
Select the [Box] tab in the right panel.
If the panel is closed, open the right panel.
Enter a value in the [Border Radius] field under the [Appearance] menu to set the border radius.
Click the icon to the right of the value field to switch between editing all corners at once or each corner individually.
Helpful Shortcuts
Hold Shift while dragging the anchor point to change the value in increments of 10.
Hold Option (Alt on Windows) while dragging the anchor point to switch between editing all corners at once or each corner individually.
Making an image a perfect circle
Setting the border radius to 50% turns an element into a perfect circle.
border-radius radius round circle ellipse rounded