how to generate electricity from down flow of water from multistory building over head tanks?

Answer 1

Hi. The most efficient way would be to bypass the power to the pump used to raise the water in the first place and use it directly. This would eliminate friction losses and generator losses.

Answer 2

The same way that a hydroelectric dam does. The water flowing downward under the influence of gravity can drive turbines which run generators. You won't get a whole lot of power this way, though, because the volume of water is relatively small. Usually water tanks on top of buildings are there to increase the pressure of the building's water supply, or to act as dynamic dampers to reduce movement from wind and other factors.