You can do this from the admincp, under Maintenance > Execute SQL Query, and select "Weekly Email Notification" from the drop down menu. To execute an SQL query from the admincp, your userid has to be listed in the config.php, in the $config['SpecialUsers']['canrunqueries'] variable.
If you want to use phpMyAdmin instead, the query is:
Code:
UPDATE user SET autosubscribe = 3