Go to the city screen, and if you click on one of your citizens faces you will see what square he is working. Now if you click on one of those squares you will take the citizen off and turn him into an entertainter. Click on the entertainer face to change him to a taxman, click again for a scientist. It's simple when you know how but I remember back in my civ1 days when at first I didn't know about entertainers. Civil disorder was a disaster as there was no way of getting the city back in order again.