You should be copying the rez files to another folder and modifying them there. You need to rezextract the rez files, which generates a folder of whats contained in the rez file. then you modify the files in the extracted folder. When you are done you rezinject them into the rez files in the game folder.
Follow the instruction in this thread HERE
Always copy the rez files to another folder called 'Original rez' so you have a clean file in case you screw up!