Hexadecimal is a system of counting, there is no "English hex" or "Japanese hex".
If you want to extract a Japanese script then you need a Japanese table file, that is a table that specified what character each hex value equals. A script extractor then looks at the ROM, sees a particular hex value and spits out the equivalent Japanese character. The result is the script from the game (usually accompanied by a lot of random control codes and other values that appear in the script but aren't text.)
Script extraction has nothing to do with changing a particular hex value from Japanese to English. That's done purely with a hex editor.
Let's say the hex value 0x40 corresponds to the Japanese character あ (which would be displayed as 40=あ in a table file.) To "translate" that in the game, you open the ROM in a tile editor, find the tile that contains the character あ and draw (or cut & paste) an A over it.
Script extraction is used to make it easier to translate the text for a translator. You use a script extractor and Japanese table file to extract the Japanese script, then you edit the font to include English characters (unless you're working on a game that already includes them), build a new table that features your new English characters and then use a script inserter and the new table to reinsert the script. (Of course, it's much more complicated than that because you can rarely ever simply copy the English script over the Japanese script. It usually requires copious amounts of formatting, pointer hacking and possibly even more complicated techniques, depending on the game.)
If I were you, I'd start out trying to extract the script of an English game, just to get the hang of it. Once you can do that okay, then try a Japanese game.