but what I really want is that the input "DE" is written to EEPROM (I can't provide "0xDE") This is because I get the value from a FORM submitted via a webpage. Now I realise I have a problem when reading the values from that FORM. I'll have to solve those first and then this problem will be solved too.
Yes, you will have to write some code to convert a string like "AD" to a byte, so that you can write it to EEPROM.
It really isn't that difficult. Extract each character. If the character is between '0' and '9', subtract '0' from it. If the character is between 'A' and 'F', subtract 'A' and add 10. If the character is between 'a' and 'f', subtract 'a' and add 10.
If the input is "AD", the characters are 'A' and 'D'. The values that you end up with are 10 and 13. Multiply the first by 16 and add the second. 10 * 16 + 13 = 173. 173 is equal to 0xAD.