Do anyone of you know on how to replace a bearing on BB7800 ? Do I need a special tool ? or a screw driver, and hammer would be enough to push it out ?
This Park Tool web page shows the basic procedure.
You need the BB cup tool for that particular model of cartridge.
The Dura-Ace BB-7800 is designed to be replaced as a cartridge unit. Replacing the bearing in the cup, without replacing the cup, would be difficult, if not impossible without damaging the cup.
These bottom brackets are exceptionally cheap, and really don't have much to them beyond the bearing, and the threaded cup to hold it in the frame.
Why spend the time and effort to replace only the bearing?
If you want to replace the whole cup cartridge, you need a 5mm allen key, a 3/8" drive torque wrench capable of 40Nm, a 1/4" drive torque wrench capable of 12Nm, The Shimano Hollowtech 2 BB cup tool, and the Hollowtech 2 BB bearing adjustment tool.
Procedure here from Shimano on PDF.
Video of general procedure here.