How do you replace the thermostat located on a 1998 Honda Prelude?

Try looking at the lower radiator hose. It should end up somewhere on the block. In any normal car, that is where the thermostat housing is located. my guess is there are two bolts on either side of the T.H., and a red, blue, or black ring that runs along the base, which is a silicone gasket. remove the bolts and pry the housing from the block (be careful). the thermostat should be seated inside the housing. when you install it make sure its pointed in the right direction.

Slightly more precise answer....

The thermostat housing is on top of the engine at the transmission end. The thing you're looking for is silver and has one radiator hose, two sensors and a bolt with a hole in the middle of it (the bleeder bolt) sticking out of it. The left end of this assembly is held in with two bolts, like the first responder said. Use a 12mm wrench to loosen the bleeder bolt, then use a 10mm socket to remove the two retaining bolts. Pull the assembly (we'll call it a cup) away and you'll find the thermostat in the piece that's in your hand. Look at how the thermostat's sitting in the cup, then remove it.

Put the gasket on your new thermostat, and set it in the cup the way the old one was in there. There's a pin on one side of the thermostat, and it should be at the top. Put the cup back in its mount, thread in the bolts and tighten them to 9 lb-ft, or 108 in-lb if you've got a 1/4" drive torque wrench.

Now remove the radiator cap and pour some coolant in there. Use either "all makes all models" coolant or Honda Genuine coolant--nothing else. When coolant comes out of the bleeder bolt, tighten it and replace the radiator cap.