Edit: The latest update is now Exchange 2013 Cumulative Update 18. See here for a list of all updates and KB articles.
I’ve got a few Exchange 2013 installations in production now so I thought it was about time I wrote a post on best practices when installing Exchange 2013 Cumulative Updates and Service Packs.
Each Cumulative update is a version of Exchange in it own right. Therefore if you are installing a new deployment of Exchange you can install straight from latest Cumulative Update, which at the time of writing is CU11 (released December 2015). KB3099522 contains a list of fixes and enhancement for CU11. Check here to see what the current latest version is.
This also means that once installed you cannot uninstall a Cumulative Update, if you do you uninstall Exchange.
The update is approximately 1.6GB in size and can be downloaded from here.
In my test lab with a single Exchange server and single Domain Controller the update took an 60 minutes to install.
The official Exchange Team blog post can be found here.