Concerning the update/upgrade, I suggest that you read the chapter 8 of the user manual.

In very summary, an update means that only the PHP code is modified and that the DB structure remain the same. So you can perform the update once on the master and this will affect all the slave sites.

An upgrade mean that the PHP code AND the DB structure are modified.
In this case, you also have to apply the upgrade in each slave site to apply the DB modification structure on each slave and to be compatible with the new PHP code.

If you want to be able modify a content in one website and be reflected automatically in another website, this mean that you want to share the content of this extension.
Decide to share the content of an extension is a decision that must be taken when creating a slave site.
See the list of extension that are currently defined in the JMS tool menu and that can have their content shared.
www.jms2win.com/en/list-of-extensions?vi...id=269&Itemid=53