Exchange Server 2013 RTM Cumulative Update 1

Microsoft has released Cumulative Update 1 (CU1) for Exchange Server 2013 RTM which is the minimum  required version of Exchange 2013 for on-premises co-existence with Exchange 2007 (SP3 UR10) and Exchange 2010 (SP3), you can download from Here.

it include some improvments to the Exchange 2013 and also changes in AD permissions and schema update.

Read the KB before applying the update for more details.