BuddyPress.org

The safest way to do it is like this:

– If your site is currently being actively used, block access with an .htaccess rule. (This is not required, but I do suggest it.) See http://www.techiecorner.com/97/redirect-to-maintenance-page-during-upgrade-using-htaccess/ for example
– Download the most recent stable build of BuddyPress https://downloads.wordpress.org/plugin/buddypress.1.2.8.zip and unzip.
– With an FTP program, go to wp-content/plugins. Delete the buddypress directory and its contents (Don’t worry – this only deletes the plugin files. Your data is stored elsewhere, and is completely safe.)
– Upload the unzipped buddypress directory to your wp-content/plugins directory.
– Verify that the site is working, and then remove the redirects from .htaccess (if you did that step).