BuddyPress.org

check your language-files at
……/plugins / buddypress / bp-languages

There should be a .pot file which you open with a software named “POEDIT”. Just Google for this software and download it for free.
Then change the wording to whatever you want.
Then you save the customized file with two different format-extensions (.po and .mo) and upload both of those files to your server (into the same folder where the .pot file is).