- Open one of your cities.
- In the little map of the city radius, click on one of the squares that is being "worked" (preferably the square that gives the least amount of food/trade/shields). You'll notice one of the citizens turns into an Elvis.
- Click on the Elvis to turn it into a tax collector, click on the tax collector to turn it into a scientist, click on the scientist again to turn it back into an entertainer.
If you want to put your specialist back to work, click on a square in the city screen that isn't being worked on. If all your citizens are at work, you can also click on an empty square to make Civ2 pick the most productive squares for you.
I don't think you should ever really need any tax collectors, though. They're usually a sign you're not managing your empire very well.