Okay this is one way of upgrading to v2 of recaptcha.
Go here https://github.com/google/recaptcha and scroll down to Direct Download (assuming you don't use composer) and download the package. Once downloaded insert it into: core/lib/recaptcha-master
Once you have done this go to core/modules/registration/registration.php
# Replace or comment out
require_once CORE_LIB_PATH.'/recaptcha/recaptchalib.php';
# With
require_once CORE_LIB_PATH.'/recaptcha-master/src/autoload.php';
In that same file, scroll down to VerifyData
# Replace all code relating to captcha check with
// start the error array as false, we dont have any errors yet.
$error = false;
// set the configurations for the service to work.
$secret = Config::Get('RECAPTCHA_PRIVATE_KEY');
$remoteIp = $_SERVER["REMOTE_ADDR"];
$gRecaptchaResponse = $_POST['g-recaptcha-response'];
$recaptcha = new \ReCaptcha\ReCaptcha($secret);
// Call the google service and verify the answer provided.
$resp = $recaptcha->verify($gRecaptchaResponse, $remoteIp);
// If Response was a success
if ($resp->isSuccess())
{
// No errors; continue
$this->set('captcha_error', '');
}
else
{
// Set error as true and show error on front-end
$error = true;
$this->set('captcha_error', true);
}
And finally on your registration_mainform.tpl
<div class="g-recaptcha" data-sitekey="<?php echo Config::Get('RECAPTCHA_PUBLIC_KEY'); ?>"></div>
<script type="text/javascript"
src="https://www.google.com/recaptcha/api.js?hl=en">
</script>
<!-- Recaptcha V2 -->
<?php if($captcha_error == true)
echo '<p class="error">Please verify you are human!</p>';
?>
you can make a function for that like v1 but that's up to you. Let me know if this helps you!