[Sagar Jadhav]

Try following line of code. Add it in functions.php in your theme.

add_filter( 'bp_get_directory_title', 'change_activity_title' );

function change_activity_title($data) {
	if( $data == 'Sitewide Activity' ) {
		$data = 'Name';
	}

	return $data;
}

Let me know whether it is works or not.