You have two options:
install phpMyAdmin and enter that text into the query box and run it
OR
create a new file called something.php and enter the following code into it:
PHP Code:
<?php
include "global.php";
$result = $DB_site->query("UPDATE user SET receivepm=1");
echo "successfully executed query";
?>
Now copy that file into the same folder as your forums and visit it once with a web browser. If the page returns 'successfully executed query' then you have just altered every user's profile.
Then
delete the something.php file.