I was given a 21 gear mountain bike as payment for some work I did for a neighbor which had been in his backyard for about a year. Though it cleaned up very well there is one part I would love to replace: On the back rim, there's a plastic piece separating the seven gear sprocket and the spokes. It's pretty beat up.
Is it hard to replace, and can you tell me how? I'm no bike mechanic, but have gotten my hands dirty.