The Cumulative Update 6 package can be used to perform a new installation of Exchange Server 2013 or to upgrade an existing Exchange Server 2013 installation to Cumulative Update 6.

You do not have to install Cumulative Update 1 or a later cumulative update for Exchange Server 2013 when you install Cumulative Update 6.

For more Info: Issues that the cumulative update resolves

Download: Cumulative Update 6 for Exchange Server 2013