Sounds like an unlucky one there!
When you say the case do you mean the bottom half of the flash rather than the bounce head?
First things first you would need to discharge the main condenser (a huge capacitor) through two holes on the right hand side of the head when you remove the rubber round hinge cover.
If you cannot discharge the main condenser then I would not continue as you have a very high risk of giving yourself a serious shocking and at the same time damaging the flash circuitry.
If it is the bottom cases that needs replacing then first remove the four screws you can see holding the foot mount on and very carefully removing the connectors as you take the mount off, what ever you do dont try to yank them as they are pitifully weak and tear easily.
Then you turn the flash head sideways so that it would be facing the roof but rotated so the beam would be infront and behind rather than to each side of you. you should see some more screws (4 screws, 2x on each side) then open the battery lid, press the bounce head lock pin and seperate the cover paying attention to two hooks at the top front.
That should give you a start on getting inside it but unfortunately the back cover is much further on and needs a lot of parts to be taken out to get to.
The front cover also needs about 12-15 parts taken off to be put on a new one so as far as repairs go it wouldn't be a case of pop each case off unfortunately.
As for the parts the front cover is available on ebay
(literally just the black plastic without even the red clean plastic cover) for about €25-6 with shipping but the back (not sure of price but would probably be ~€30 judging by the front) is not available there and Nikon will most definitely not supply you with one even with the correct part number as they are very strict on the parts front to everyday peopel and also being a flash will not "be responsible" for supplying a part with the risk of shocking or damaging the flash. The same goes for any circuitry pretty much on the bodies and lenses.
I would very much not recommend doing this yourself an getting it repaired as there is such a high risk of causing injury as well as damaging the flash more apart from the procedure not being straight forward just removing the plastic and popping a new one on.
If you didnt want to get someone to repair it could you post a picture of the damage perhaps as if not bad you could possibly epoxy cracks if they weren't too bad and just use the flash (even if it looked battered) if it is operating correctly.
Hope that helps!