Once your in the city screen, click on the gold, wheat, etc. icons within your city radius. You'll notice citizens turning into entertainers. Then you can click on the entertainer to make him, or her, a tax collector or scientist or back to an entertainer. Hope this is what you wanted.
Thanks, but that's not actually what I meant. If you go to the city display and hit 'G' it brings up the Governor's Display. From here there's an option for 'Managing Citizen Moods'. I'm not sure how this option works.
I believe that when the Governor takes over for citizen moods, or anything else for that matter, he actually does the switching to entertainers (when managing moods) for you.
Whereas, as has been mentioned, you can do it yourself on the city screen.
Actually, I found that doing it yourself is best. The governors have a habit of making some really dumb decisions. For example, taking one of your more productive tiles and switching that to entertainer, instead of a weaker and equally unhappy one.
It can, though, help those people that do not want to micromanage their cities.