Change shape to circle I am doing this Blenderguru tutorial to work on my modelling skills. 



Question At about 28:30 min in the tutorial it is demonstrated how to assign a material to a selected parts of the mesh. It should become a black circle and a white circle (green in my image). But I guess my shape is not a circle. I think the reason for that is this loop cut I made and that is not there in the tutorial. 



But how to change my shape to a circle ? I have tried deleting edges but that does not work.
Just select the edge loop (Alt +RMB). You will need to scale it flat if it isn't already, and since from your image it looks like "Y" is your local axis, scale it to zero on the y S > Y axis. In my image, "Z" is the local axis, I will scale to zero on the "Z" axis. 

Before Scaling:


After scaling:


Now use the "to sphere" tool. since it only has two dimensions, there won't be any sphere going on, just circle.

you can find this tool with Spacebar and then searching or Shift+Alt+S.