First of all you have to edit the wp-config.php file of your WordPress site and increase its PHP memory limit.
It is present in the root folder of your WordPress site. To edit it, you have to login to your web hosting control panel.
Now in your wp-config.php file /* That's all, stop editing! Happy blogging. A code has to be pasted before the */ line, which you can see below.
define( 'WP_MEMORY_LIMIT', '256M' );
Now this code will increase your WP memory limit up to 256MB.
Now you can visit your WordPress website to see if WordPress memory exhausted error has been fixed or not.
If fatal error allowed memory size exhausted is not fixed then it means that your PHP memory limit has not increased or web hosting provider does not allow to increase it. For this you have to contact your hosting provider.