How to add/replace doublequote in C# ?

string str_Add_Doublequote = "<p style=\"font-size: 10pt; font-family: sans-serif;\"><font class=\"Apple-style-span\" face=\"Tahoma\" size=\"2\"></font></p>";
string _Double_To_Singlequotes = str_Add_Doublequote.Replace("\"", "'");
string _single_To_Doublequotes = _Double_To_Singlequotes.Replace("'", "\"");