If the range is drastick like 0-3000, then you need coolant for sure or you have a coolant leak somewhere within your EACV valve and your Thermovalve, check those hoses, I know on my 94' Prelude there are 4 hoses between the two components.

Just remove the cap on top of the radiator "not the overflow tank cap". Pour in radiator coolant and squeeze upper radiator hose to push the air out of the radiator, keep doing that till you can't force anymore air bubbles out of the coolant system to suck down anymore of the coolant you are trying to add, then simply tighten the radiator cap back on and start up the car should be good again.

If the range is little, such as 0-1300 rpm's, then simply check your oil "make sure you do the check when the engine is cold" If you try to check it right after you ran the car, the oil will be hot and read incorrectly telling you that you have somewhere around a 1/2 a quart more than you really do. If you had enough oil then check your idle screw, try turning it to the right till your rpm's drop, but if you get to the end and it is still doing this small rev, then you could have advanced timing, one of your accessory belts could be either far too tight or even a little bit too loose, your throttle cable could need adjustment or replacement "if there are any extreme bends or kinks in the cable you need to have it replaced. There could be a small vacuum leak somewhere around your intake manifold, and there are another very rare instances that it could be but I would start with all of those first, then if that doesn't fix the problem just let me know and ill be sure to try to help ya out. Anyways goodluck guys I know how annoying the sound of engine hi-rev is but with patience and a little adjustment it will go away.