Note that you are talking about the (canonical) string representation of a Guid. The Guid itself is actually a 128-bit integer value.
You can use the “N” specifier with the Guid.ToString(String) overload.
Guid.NewGuid().ToString("N");
By default letters are lowercase. A Guid with only uppercase letters can only be achieved by manually converting them all to uppercase, example:
Guid.NewGuid().ToString("N").ToUpper();
A guid with only either letter or digits makes no sense. A guid string representation is hexadecimal, and thus will always (well most likely) contain both.