This update rollup is highly recommended for all Exchange Server 2010 customers.
For a list of changes that are included in this update rollup, see KB2407113.
The is only one fix:
You cannot uninstall Exchange Server 2010 RTM after you install Update Rollup 2, Update Rollup 3, or Update Rollup 4 for Exchange Server 2010
Download the rollup here.
Note to Forefront users:
If you don’t disable Forefront before installing a rollup or service pack, and enable afterwards, you run the risk of Exchange related services not starting. You can disable Forefront by going to a command prompt and navigating to the Forefront directory and running FSCUtility /disable. To enable Forefront after installation of a UR or SP, run FSCUtility /enable.