Escape double quote character in XML
Others have answered in terms of how to handle the specific escaping in this case.
A broader answer is not to try to do it yourself. Use an XML API - there are plenty available for just about every modern programming platform in existence.
XML APIs will handle things like this for you automatically, making it a lot harder to go wrong. Unless you're writing an XML API yourself, you should rarely need to worry about the details like this.