How to change multiple file names in C#?

I have file names that contain numbers that need to be decremented by 1. I need code that satisfies the following pseudocode:

class ReplaceSubstrings
    {
        string searchFor;
        string replaceWith;
        Bool filenamecontainnumbers; 'file name contains numbers to be       decremented by 1
        int filenamenumbers; ' file name size varies from 10 to 500 - I only want to          ' target integers 200 - 500

    while(filenamecontainsnumbers = 1) ' maybe I should use Bool condition

    ReplaceSubstrings app = new ReplaceSubstrings();
    If(filenamenumbers > 200){
        app.searchFor = filenamenumbers; // A very simple regular expression.
        app.replaceWith = decnamenumber();
        Console.WriteLine(s);

    return 0; 

}


void decnamenumber(){

filenamenumbers = filenamenumbers - 1;

 };

or something like this!!! Help Please!!!