5.63 is the Extended-Security-Release version, which is a subscription service. You can sign up for the ESR service & download that version for a small fee. Read more about it here: https://civicrm.org/blog/kbeireis/esr-long-term-solution
If you do not wish to sign up for extended security releases, you can download the latest version of CiviCRM for free, which contains all security updates.