Editing exsisting data

I have been looking at the maps for my local area some of which I know very well, there are some errors already made is it possible to correct these errors? if so how?
The area is question someone has drawn 4 mini roads in a diamond shape and labeled them as a roundabout, this part of the road is just a normal 3 way juntion and the fourway roundabout is 1/2 mile futher up the road and already maked as such.

I think the Beginners Guide will help.