Hello, I have a rounded head, actually in a bad way, my forehead looks
bad and my head formation is not natural (born that way). What should
I do to decrease the bad rounded shape of my forehead?

I think that if changing the shape of your head is possible, it is
probably dangerous to your brain.
I suggest trying various hair styles, I'm sure you can make your face
look more the way you'd like with the right hair doo.

Beyond that, I am afraid that there isn't much you can do about at
this stage in life.

This site mentions that several cultures have deformed their heads,
but always starting in infancy:
http://www.sacred-texts.com/atl/ataw/ataw309.htm