how do u take the rear drum off a 1991 corsica v6

Here is a link that shows images and gives steps on how to take  the rear drum off your car: http://www.diynetwork.com/diy/ab_brakes/article/0,2021,DIY_13682_2276024,00.html

You may have to back off on the brake adjustment (thru the slots on back of the drum plate) Sometimes if the drum is worn it makes a lip inside and if the brakes are adjusted out far enough to work it will be too tight to slide off after everything else is loose. Sometimes you just have to use a big hammer and tap around on it (carefully)to break it loose.

On older cars you might have to use a brake drum puller due to rust an/or that "lip" that Corollanut is talking about. You can probably rent one or use one for free from a chain parts store if you buy their parts.