how do I change water pump on 1990 Honda Prelude 2.0SI

If your water temp gauge shows normal and your sure engine is running at normal temp and the water pump is not leaking I would not change water pump. I would verify that the engine is running at normal temp. There are several ways you can do this. There are radiator caps that have a built in temp gauge. You can use an infra-red thermometer and check engine temp right on thermostat housing. Also if your familiar enough with your engine you should be able to tell from experience what your engine" feels like" when up to temp. If you are 100% certain that the engine is at correct temp you will need to look at electronic controls for fan. Some things to look for .....stuck fan relay, bad temp sensor or wiring issues. If you get to this point you might want to get a Haynes manual for your car and work through the wiring schematic.

Fan relays is correct - most common reason for the problem - although mine was a rare case of getting stuck at "never run" instead of 'always on' and led to overheating. For changing the water pump: First locate it (here's some help: http://www.preludeonline.com/f28/water-pump-location-250535/) and then it's a simple enough straightforward process if you know what you're doing. Step by step instructions on how to replace the timing belt and water pump for a 1995 Honda Prelude DOHC VTEC used to be here: http://www.youtube.com/watch?v=iiE1s5tcpeQ but the user seems to have removed it. There's another multi-video instructional for Honda 4 cyls, hope you find it useful: http://www.youtube.com/watch?v=AN2kXUvwkng Else you can get your belts and pump changed in a day if you drop the car off early enough and should cost you around $650-$800 at a decent shop.

bad thermostat or sensor ?