use ftp, go to the languages folder, pick your language, most of the time it will be the common.php file.
only use a good text editor. not word, not dreamweaver etc. a text editor only.
find the relevant listing, they are alphabetical,
in the case of the private message one it would be this line:
'PRIVATE_MESSAGES' => 'Private messages',
edit the right side. leave the single quotes don't use any apostrophes.
be very precise in your edits because even one tiny type could make the whole board not work properly.
make backups before you start.
I am available for custom work on a donation basis. Please send me a PM with your needs.
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OK, so what's the speed of dark?