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CCAD's AutoCAD Tutorials
How to Edit a Circle Using Multifunctional Grips
Make sure Dynamic Input (F12) 
is on to view the radius of the circle.
Select a circle and place your cursor over one quadrant of the circle. Don't click, just hover over the grip. Note: The center grip will only MOVE the circle.
The numbers will be formatted in accordance with the settings in the UNITS command.
Click the grip and move your cursor. The center point of the circle doesn't change.
Click to specify a new point on the circumference of the circle or type a number in the highlighted input box and press ENTER to specify a new radius for the circle.
Press the TAB key to change the size of the circle by adding or subtracting an amount. Move your cursor and type a number in the highlighted input box and press ENTER. If you move your cursor outside of the circle, it will become larger. Move it closer to the center of the circle will become smaller.
Any dimension objects attached or associated with the circle will display the new size.