How do I send an email when users update their email address?
You could add a HOOK_user_presave method in your custom module.

Example:

function mymodule_user_presave(\Drupal\user\UserInterface $user) {
    if ( !($user-&gt;isNew()) ) {
        //check if the value in the db is different to the value in $user
        if ($user-&gt;original-&gt;getEmail() == $user-&gt;getEmail()) {
            // send email
        }
    }
}