What can I do to re-shape my baby’s flat head?

Proper Positioning. Since 1992, when the usa started putting babies to sleep on their backs, there has been an increase in temporary flattening of the head (plagiocephaly). This resolves with time and may take years. Helmets are rarely recommended as the don't provide much benefit over waiting. Limiting time on the back when awake is most important.
Skull. Make sure that you change your baby's head position while lying in the crib.