It is in the lang files.
If you are using English (en) then you would go to Admin, then Language, then Edit Strings. Those words are located in the moodle.php file. The files need to be writable for it to work.
The better solution is to copy the language folder you are using, rename it to something else (en becomes en_me) and change those files. Reset your site to your new custom language files and you never have to worry about updates overwriting your work.
Jeff