Issue: You need to confidently upgrade an existing Exchange 2013 environment to the latest version of Exchange 2013, CU21.
According to Microsoft you can upgrade from any version of Exchange 2013 to Exchange 2013 CU21.
Upgrade to CU21 Step by Step:
1) Follow this link to download the latest version of Exchange 2013.
Note: if you are running a DAG, review this article: How to install Exchange CU updates on a DAG.
2) Execute the downloaded file on the first Exchange server you will be upgrading to CU121. I recommend extracting to an easy to type folder, like C:\Exch2013-CU121.
Note: Be mindful of hard disk space, it may make sense to extract to a folder on a disk other than C:.
3) Next, CU21 may require that you prepare AD and upgrade the AD Schema for Exchange 2013 CU Upgrade. That link will help you determine if it’s necessary for you to run a Schema upgrade from the currently installed CU as well as walk you through the process.
4) Open an command prompt using right-click-run-as-administrator
5) change directory to the folder where you extracted the CU21 files “cd\Exch2013-CU21”
6) type “setup.exe”
7) click ‘next’ on the connect to check for updates screen
8) Once the update portion is complete file copying will begin
9) Setup will detect this is an upgrade, click next to confirm/continue:
10) Next, choose whether you accept the license agreement
11) The Readiness Checks will now occur
12) Fix any issues found in the Readiness checks and click retry, repeat this process until the readiness checks complete successfully
13) The upgrade will now proceed through it’s steps, starting with Organization Preparation
14) The upgrade will take quite some time, and then once it’s complete it is recommended you reboot the server.