Generally babies sleep their heads flat if the skull development is set, it may be difficult to change into a round head, but if it is still in the developmental stage, need to try to change the sleeping position, may be helpful.
In general, when the baby is found to sleep flat head, parents can usually help the baby turn over, change different sleeping position, avoid long time with the same position to one side, take alternate side lying position to sleep, to a certain extent may be able to change the sleep flat head shape, head shape to become round has some help.
However, for babies with closed fontanels, since the head shape is basically set at this time, it may not be possible to change it.
All in all, when you find that your baby has a flat head, you need to go to the hospital and work with the doctor to improve it.