Try:
myString = myString.Replace("\\\"", "s");
Or
myString = myString.Replace(@"\""", "s");
Either should do it
Or (if you are allergic to String.Replace perhaps) then there are always Regexes:
myString = Regex.Replace(myString, "\\\\\"", "s");
If that isn't what you want, then you need to explain in more detail.