We released version 1.8.6 in December. An email notification was then sent to all licensed users, where a link to the official announcement was included:
https://pixinsight.com/mail/20181216-1.8.6-release/In the announcement, I describe the new licensing system in detail. You should have received that email. If you didn't receive it, that's because either you disabled email notifications in your user profile, or because your email client application and/or your email provider have blocked our emails or filtered them out as spam. We are not responsible in any of these cases.
On the other hand, if you were using version 1.8.5, you should have received a notification about the release of version 1.8.6, where a link to the official announcement is also included, along with a message that informs you that you have to install the new version manually.
Finally, see FAQ section 3 for more information about installations on different platforms:
https://pixinsight.com/faq/