C# - How To Put Escape Before Special Characters

buildLetter.Append("</head>").AppendLine();
buildLetter.Append("").AppendLine();
buildLetter.Append("<style type="text/css">").AppendLine();

Assume the above contents resides in a file. I want to write a snippet that removes any line which has empty string "" and put escape character before the middle quotations. The final output would be:
buildLetter.Append("</head>").AppendLine();
buildLetter.Append("<style type="text/css">").AppendLine();

The outer " .... " is not considered special chars. The special chars may be single quotation or double quotation. I could run it via find and replace feature of Visual Studio. However, in my case I want it to be written in c# or VB.NET