You don't need any tool at all though you may need a small file as they are tight sometimes and go in one way so try both ways before filing. A few slight file stokes made mine work, did not even remove any finish from the sight (which would have been hidden anyway)
OK, Here it the trick but watch out as a spring may fly out. Either back the sight screw out all the way but I find this much easier. You see the pin that the rear sight screw rests in, the pin is spring loaded. Just find something that won't scratch the gun. Something small and plastic like a pen cap, push down on the pin and slide the sight out. This is when the spring will fly so be carefull.
Oh and if you got Meps, the two pair I got both had instructions that said if the rear was tight, file on the slide. Considering the price of a slide compared to the price of a new rear sight that hit me as a very stupid idea. Test fit it before you put the spring and pin back in. Try from both sides, it will go in easy from one side. If it won't go all the way in once again get something that will not mar it or just push it with a piece of wood. Do it somewhat hard but don't force it real hard, tap it back out, look on the bottom of the rear sight and the dovetail sides for scuffed areas. Hit these just a few times with a fine tooth file, try again and see if you have to repeat it. It should take almost no filing, it did not even leave a silver spot on mine.