To check for the current version use the following command line

Version 15.0 (Build 516.32)

Get-Exchangeserver | ft Name,Admin* -Autosize

How to upgrade your existing Exchange Server 2013 to CU7 using command-line

You will have to download CU7 pack, extract it and run the command line from CMD with administrative privileges.

http://www.microsoft.com/en-us/download/details.aspx?id=45221

Here we run the CMD as admin

Drag and drop the folder you extracted into CMD window to be able to enter into the path in order to run the setup file.

Run the following command to upgrade the existent server

Setup /Mode:Upgrade /IAcceptExchangeServerLicenseTerms

Below you can see the upgrade process to install the Cumulative Update 3.