BuddyPress.org

Your query throws an error. Did you check your logs?
And you don’t need theglobal $bp

global $wpdb;
$wpdb->query(
    $wpdb->prepare(
        "UPDATE {$wpdb->prefix}bp_activity
        SET content = replace(content, %s, %s)",
        'thing1',
        'thing2'
        )
);