I Have Textured Plaster. How Can I Remove Cover It?

Textured plaster can be sanded smooth and re-plastered, though this work is very labor intensive and messy/dusty!  I had three major walls sanded this way in a remodel, and the fine dust is everywhere-so be sure you remove furniture from the room and allow for good ventilation.  Be cautious about sanding, also, since some older building materials contain asbestos.
 
Removing the old drywall is the next best thing.  Depending on the degree of texture on the current wall, it is probably impractical to plaster over them without first sanding.

 Textured plaster is impossible to remove.
You would have to demo the walls and hang new drywall.
The only other thing I can think of that you could do is to hire a Plasterer to come in and smooth the walls out with fresh plaster.
Sorry to give you that kind of news. If the plaster design isn't too thick, you might could have a drywall mason to spray over it with texture for a knockdown or orange peel effect.