Availability depends on your subscription status.
If your update subscription has ended before 2021.3 was released, there's no way to update your SAM to that version.
If your update subscription includes 2021.3 and/or newer versions, you should alwyas be able to download the latest version covered by your subscription in the user panel.
As you can see from this Screenshot, my subscription expired in 2020 and thus I would not be able to download the newest SAM version (or any that has the fix):
Screenshot 2021-11-24 023020.png [ 11.29 KiB | Viewed 250 times ]
The update process is as simple as installing a new SAM version on top of your old SAM. It should keep all the settings and your music library untouched.