change email verification body

You can override the sendEmailVerificationNotification function from the MustVerifyEmail trait on your user model by adding you own version of this function in app\User.php

    public function sendEmailVerificationNotification()
    {
        $this->notify(new Notifications\VerifyEmail); // Replace this with your custom notification class
    }

Details on creating notifications are in the official docs

https://laravel.com/docs/5.7/notifications#creating-notifications

You can use a different notification class as suggested, but if you look further, you'll see that the email verification notification is just pulling the text from language files.

    public function toMail($notifiable)
    {
        if (static::$toMailCallback) {
            return call_user_func(static::$toMailCallback, $notifiable);
        }
        return (new MailMessage)
            ->subject(Lang::getFromJson('Verify Email Address'))
            ->line(Lang::getFromJson('Please click the button below to verify your email address.'))
            ->action(
                Lang::getFromJson('Verify Email Address'),
                $this->verificationUrl($notifiable)
            )
            ->line(Lang::getFromJson('If you did not create an account, no further action is required.'));
    }

So you'd just need to change the text in the language file, which you can read about here: https://laravel.com/docs/5.7/localization#using-translation-strings-as-keys

/resources/lang/en.json

{
    "Verify Email Address": "Use this subject instead",
    "Please click the button below to verify your email address.": "Other text instead",
    "If you did not create an account, no further action is required.": "Use this text for the button instead"
}

@Cronix - the one annoying thing is that the same key is used for the subject and call to action text

Lang::getFromJson('Verify Email Address')

Therefore you can't change them independently without a custom notification