The latest version of Exchange 2013 is currently 2013 CU15, also known as Cumulative Update 21, the next and final release being CU22 which will likely come out next quarter.
You can install Exchange 2013 directly from the CU15 media, which is much better than installing the base RTM and then subsequently upgrading to Exchange 2013 CU15 on top of that.
Here is a link to download 2013 CU21 from Microsoft.
If this is a new install, make sure you have installed and configured the Exchange 2013 Pre-Requisites. On any new install or upgrade of Exchange 2013 server CU15, you may need to first upgrade the active directory schema and configuration for Exchange 2013 CU21.