I need to update to Indesign CC 9.1 from 9.0. How to i do that? I can't find anything.
There are two ways to update.
(1) From within InDesign, choose Help > Updates.
(2) If you've installed the Creative Cloud Desktop app (which you can download via creative.adobe.com after you login with your Adobe ID), you'll be prompted for updates.
How can I manage the document compatibility between this new release and InDesign CS3 we are now using?
Is it possible to open and share between diffent workstations the new documents we could create with the new version with the old one?
There is no good way to share files between CC and CS3. The ONLY possible way is to export from CC as .idml, open that in CS4, and re-export as .inx, and the .inx, when opened in CS3 will probably only vaguely resemble what you have in CC.