Where is and how do you replace the speed sensor on 1992 Honda Civic DX?

I have a Honda Civic 99 LX 1.6 Aut, and the D4 is Flashing and the gears doesn't shift at all, some people say that the speed sensor e some others don't even know what the heck it is... Does anyone with more knowledge than me could help me with this. How can I know if the speed sensor is dead, is there anyway to find out?

And where the speed sensor is located so I can remove and see it? Thanks a lot in advance. Ivan