Ground bus

I have a 200 amp Square D main panel and recently I found that some water had been seeping into the panel through my main cable from the meter. That has been repaired. However, the ground bus (I guess that's what it's called)on the left side of the breakers has become corroded. Do I need to replace it and if so how do I do that? So far as far as I can tell everything in the house is working fine. Must I turn off the main breakers before I remove the old bus? I have done quite a bit of electrical work but have never done something like that. Thanks. Rich