Square to circle...

Hello All!
I’m trying to modelize a cube but the top of the cube has to be a circle
From that picture, how to make (step by step if possible) the yellow square to become a circle?
http://88.191.30.201/%7Etukuyomi/stuff/blender_square_to_circle.png
Thank you in advance (btw I’m a newbie :))

Use the ‘To Sphere’ tool.

In Edit Mode, Mesh -> Transform -> To Sphere (CTRL+SHIFT+S)

Ah, Thank you =)
It was that simple after all ^^