If you use a build from ardour.org it will notify you when there is a new release. The dialog that will appear features a button you can click that takes you to the download system.
I send out email to the ardour users mailing list when there are new updates. You can also point an RSS reader at community.ardour.org to pick up release announcements.
aha. Work not done: I never set the new release in that system (waiting for feedback, which was wise, since 5.6 had a number of important regressions. 5.7 will be out on Friday. Thanks for the incentive to discover my mistake.