Modelling a head question

Hey, I finished creating a face and i filled in the rest of the head by extruding a plane and joining the vertices’s where necessary but the effect is that the back of the guys head looks like a ski jump

What would be the best way to make it rounder ? (maybe using proportional edit? if you think so you will have to tell me how as i have never used it before)

Picture of the problem is here

Thankyou in advance

Here’s one way. You’ll need a reference drawing. Google “wiki skull” for an image from Grey’s Anatomy.