From time to time, Microsoft release service packs and update rollups for Exchange. There are some things to keep in mind when it comes to applying these updates to your environment:
• Update rollup packages are cumulative. For example, if you apply Update Rollup 4 on a computer that is running Exchange 2010, you receive all the fixes in that specific update package together with all the fixes that were released in all earlier update rollup packages. That is, in Update Rollup 4, you receive all the updates that were released in the previous Update Rollup 1 through Update Rollup 3 packages plus the updates in Update Rollup 4.
• This approach to updating doesn't replace service packs for the product. Additional product fixes and features are released in separately available service packs for Exchange 2010.
• Microsoft releases update rollup packages approximately every six to eight weeks. The rollup packages are available via Microsoft Update and the Microsoft Download Center. In the Search box on the Microsoft Download Center, type "Exchange 2010 update rollup" to find links to the rollup packages.
• For Exchange 2010 configurations, the recommended order in which to apply the update rollup is as follows: Client Access server, Hub Transport server, Mailbox server, and Unified Messaging server.
To find out which version of Exchange 2010 you're running, logon to your server then go to Control Panel - Programs and Features. Highlight Microsoft Exchange Server 2010 and make a note of the Version.
Or run the following Exchange Management Shell command:
Get-ExchangeServer | fl name,edition,admindisplayversion
To see a list of Exchange versions, follow this link: http://technet.microsoft.com/en-us/library/hh135098.aspx