As noted in the comments: you should actually move/rename the folder, not just print out its name.
DirectoryInfo dir = new DirectoryInfo("C:/Users/Wiz/Desktop/folder/");
foreach (var item in dir.GetDirectories())
{
item.MoveTo(Path.Combine(item.Parent.FullName, "a" + item.Name));
Console.WriteLine("a" + item.Name);
}
Console.ReadLine();
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solved How add letter for the folders start