Microsoft Released CU9 for Exchange 2013 and recommend their customers to upgrade to this version as it has a lot of bug fixes in it. For more info on this release you can look here:
To upgrade your servers you can do so in the following order:
- Internet facing sites first.(If you have separate roles then your CAS first and after that your MBX servers)
If you are running a DAG then you need to put the server into maintenance mode first, here is a link for the script to do so:
Note that there are 2x PowerShell scripts to download, once to put the server into maintenance mode and the other to take the server out of maintenance mode.
To do the upgrade it is fairly easy, you run the setup (from the CU9 file you extracted) as administrator and then follow the prompts. You can let the GUI do the AD/Schema Upgrade or you can run it manually from an elevated prompt.
In this exercise i am going to list all the screen shots but they are all self explanatory.
Hope it helps.