Make it interactive. Drag the slider to change the shape:
Here is the code to draw a 12-sided polygon:
Graphics[Polygon[CirclePoints[12]]]
Give yourself a slider to control the number of points using Manipulate. Wrap the expression with Manipulate[...], replace 12 with the variable n, and let n vary from 3 to 12 in steps of 1:
Manipulate[Graphics[Polygon[CirclePoints[n]]], {n, 3, 12, 1}]
Manipulate[Graphics[Polygon[CirclePoints[n]]], {n, 3, 12, 1}]