How to update old CAPTCHA to use reCAPTCHA V2
Just update module via update modules page:
Your_site/admin/reports/updates/update

Follow steps and update DB.

Go to modules page and check configuration for recaptcha, there you'll put your new keys (got from https://www.google.com/recaptcha)

Note: If you update module files using FTP, remember to run update.php script.