You have to grab an allen wrench, loosen the bolt on top of the stem a few turns 'til it's raised up about 3/8". Then grab a scrap piece of wood and a hammer. Put the wood on the bolt head and hit it hard with the hammer. Once the bolt's knocked down pull stem. Twist a bit if necessary.

Why are you taking the stem out? Not necessary for a conversion.

Oh, before you follow my instructions you'll probably want to put your headset back together so you don't lose anything.