I'm not sure I understand the question. A letter is just a collection of bits, interpreted by the computer according to the context. Do you want to carry out transformations, eg for ciphering?

You could use getBytes:

byte[] str1Bytes = str1.getBytes();

str1Bytes[index] = changed value;

String str2 = new String(str1Bytes);

Is it possible to change the hex value of a letter? If so could someone change the letter 'R' to make it have the value of a '^' or something like that, or prehaps so me? Thanks!