Is a product update free?

What version of 2016 are you using?  All minor version updates for that major version (2016) are free.  The current and last version of 2016 is 6581.  You can get the full installer, manually/free by grabbing it from the product page:

https://www.acronis.com/en-us/support/updates/

 Acronis True Image 2016 for Mac (Build # 6077, Multilingual, 100.3 MB)2016-06-02 Release note

Hi, and thank you for your reply.

My version is 19.0.0.6571.

I searched high and low for the webpage for which you gave a link in your reply, where it says updates are free, but couldn't find anything.

Many thanks for your help.  I will install the update.

You're welcome.  I just realized I posted the link to the Mac version - the Windows one is here (if you didn't find it already from the main page also linked above)

 Acronis True Image 2016 for PC (Build # 6581, English, 479.7 MB)2016-10-27Release notes

Thank you for sharing this information. I had the similar doubts about about the updates. Now everything is clear. How often do you prepare updates?

I'm just a forum volunteer in the forum (all of us with MVP badges are).  

Acronis releases the updates as needed.  However, typically, there is a major release (new version like 2015, 2016, 2017) once each year around early fall which is a paid upgrade if you want it.  There has typically been a quarterly update to address bug fixes and things like that in each major version.  In between, you might get a hotfix as a separate KB article for specific items, but only the quarterly updates are identified via the app as an available update. 

Although Acronis now offers 2 years support for each version (1 year beyond the next version release), most of the application updates for an older version stop when the new major version is released - so basically, you get updates for about a year.  I think 2016 6581 was the only update I've seen them release for an older version, once a new version (2017) had already come out. I doubt we'll see any more for 2016 though.