If you carefully peel off the round stickers on the head, there are screws behind them which allow you to dismantle the head. I think they require an allen key, if my memory serves me correctly. The mechanism inside is very simple.

A word of warning though. When I attempted to fix my own head (it had got very stiff) there ended up being play in the gears after I put it back together - probably through lack of grease. I ended up sending it to Manfrotto UK for servicing, which cost about 35, but it came back as good as new.