If you get a PHP error like “Allowed memory size of xxx bytes exhausted” you have to increase your memory limit. To do so you have to open the wp-config.php file from your WordPress root directory and add the following line

define('WP_MEMORY_LIMIT', '256M');

Some hosts don’t allow these changes (or it has no effect). Often it gets overwritten by some global max values. Contact your host for more information.