You'll want to use the maneuvering node to change your orbital inclination using the pink up/down directional handles. What you'll find is that when you do this the ending orbit you get will start to tilt but it will also get much bigger. You'll then have to correct for this. Honestly it requires a massive amount of fuel to make this change and if you're interested in doing it because you're in a polar orbit around, say, the Mun, you will likely find you'll run out of fuel before getting back to the equatorial orbit you want.