From the wording of your question, it sounds like you are attempting to identify byte values in a string and then convert them - this won't work. Strings are composed of characters and each character can consist of more than one byte (depending on the encoding). In other words, the conversion from a stream of bytes to a human-readable string is already performed by the time you access a string.
Have a look at the System.Text.Encoding class. If you really do want to convert a byte stream from one encoding to another, try System.Text.Encoding.Convert().
But it would be nice to know more details about your specific task so that people can give you a more precise answer.