Two approaches come to mind. The first approach is to encode the string value. Base64 is the most common but you could also use hex encoding. Just be sure to always encode/decode so there is no confusion.
var someValue = "Hello>There";
var encodedValue = Convert.ToBase64String(Text.Encoding.UTF8.GetBytes(someValue));
The alternative is to wrap the value in a CDATA. CDATA allows you to put non-XML stuff into XML. It is most often used when trying to put things like formatted code into XML. However some characters can still cause issues so personally I would base64 encode the value.