Adobe Creative Cloud (CC) was introduced in 2013, CC 2014 was released in 2014, and CC 2015 became available last month.
Back when Adobe CC 2014 was released, subscribers that updated were left with their older copy of the CC apps. Because of this behavior, it’s become common place for some creative companies to run both CC and CC 2014 apps at the same time, because the older version proved to be more stable or compatible with something the newer version wasn’t. Updating CC apps wasn’t risky, because you could continue using the older versions.
Adobe has changed this behavior, and the Adobe CC 2015 update will (by default) uninstall older versions of CC. This means if you use both Illustrator CC and Illustrator CC 2014, and click “Update” when prompted, you will only be left with only the newer Illustrator CC 2015 app. For some this won’t be a big deal, but for many it will cripple their existing workflow.
Fortunately Adobe provides a way to update to Adobe CC 2015 apps without uninstalling the previous version(s). On the window with the “Update” prompt, click the Advanced Options section, and uncheck the option for “Remove old versions”. Read more here: http://blogs.adobe.com/adobecare/2015/06/15/creative-cloud-delivering-more-choice-for-installations/
If you’ve already updated to CC 2015 and previous versions of the CC apps were removed, you can re-install the older versions following Adobe’s instructions : https://helpx.adobe.com/creative-cloud/help/download-install-app.html
NOTE: The Adobe CC 2015 update will not uninstall Creative Suite (CS) 6, CS 5, or CS 4.