How can I change the shape of my head?

You did not mention your gender however if you have a big, bossing forehead you may easily change its appearance.  Hairline lowering by a hair transplant can change a masculine upper face into a more feminine one.As for the flat back of the head, you may want to see a plastic surgeon.

You can not change the shape of your head but you can change your hair so that the shape of your head looks different. Lowering the hairline is one example. I had a famous celebrity who had a flat back of his head and when I transplanted it pointing the hairs upward, the flatness was hidden by the hair that I placed. He loved it