Doctrinal changes often come through the magazine, specifically the "study edition". At the time of the writing of this article, there were not two editions. Now, there is a public edition (used for placing with the public) and a private one.

Some people might wonder if the study edition stuff is secret, it isn't. Its just "optimized" for the different audiences. You can find them all here: https://www.jw.org/en/publications/magazines/?contentLanguag...

At the weekly weekend meetings (typically Sunday mornings), the study articles are studied, so this information also comes through services, eventually.

Edited: for clarity around the link to the study editions.

They like to say that The Watchtower is the most read magazine in the world and that the second most distributed magazine in the world is Awake!, also published by the church