C# - Character Replacement In String?

I do need a working ASP.NET C# code to replace characters in a string.

My following code works fine but in case of input "a" it gives me an output as "678d", but in case of input "c" the output is correct as it is i.e. "8d"... Here it automatically replaces the rest values too. I can see the code is executing in a step by step process... This results me to get an overloaded output.

{
StringBuilder builder = new StringBuilder();
builder.Replace("a", "6b");
builder.Replace("b", "7c");
builder.Replace("c", "8d");
return builder.ToString();
}

Now, I do need to replace "a" as just "6b" and it should not load the rest values.