Circles created in NUBIGON and imported as editable CAD can be modified.
Launch the CAD window 
from the sidebar and ensure Viewport editing is enabled.
- To change its radius, hover over the circle edge, then left-click and drag.
- To translate a circle, hover over its center, then left-click and drag to move it on its plane.
Change attributes
- Select the circle to view its attributes in the Object Inspector.
- Modify center coordinates and radius as needed.