http://awcdev.com/how-to-change-the-notification-email-to-new-users-in-wordpress/
The notification email that new users receive from WordPress can be changed by creating a simple plugin.
First, create a plugin file in your plugins folder. Name it whatever you like (for example, change-new-user-email-notification.php).
Then, add the code below, which is borrowed from https://codex.wordpress.org/Function_Reference/wp_new_user_notification. In this example the new content for the notification email is on lines 46-50. Change those lines to suit your own needs. You can ignore the rest of the code and leave it as is.