Before applying update rollups to an Exchange server, the administrator can sometimes review the relevant KB article associated with that update rollup. This is, of course, a good thing and is entirely recommended. However, if this is, say, the second time that the administrator is about to install an update rollup, there can sometimes be associated questions and confusion on the update process. Let me explain why.
The KB article for Update Rollup 2 for Exchange 2010 SP1 is here. Towards the end of this KB article lies the Prerequisites section that says the following:
The following list contains prerequisites for the Update Rollup 2 for Exchange Server 2010 SP1:
* Install Exchange Server 2010 SP1 on the computer before you apply this update rollup.
* Remove all interim updates for Exchange Server 2010 SP1 before you apply this update rollup.
It's normally the second bullet point that causes confusion. It's quite common for an administrator to assume that they need to uninstall the previous update rollups before applying the latest update rollup. For example, if the administrator has already applied Update Rollup 1 and is now about to install Update Rollup 2, they assume that they must uninstall Update Rollup 1 as that's what the second bullet point says, right?
Actually, if you read the text closely, it says interim updates which are very different to update rollups. You only need to uninstall interim updates and not previous update rollups. Interim updates are specific updates from Microsoft PSS that have been released to customers that have called in with a particular issue; these updates are therefore designed to fix this specific issue only. You will therefore almost certainly know if you have had an interim update installed as you would have involved Microsoft in the issue resolution process.
Also, remember that update rollups are cumulative. In other words, Exchange 2010 SP1 Update Rollup 2 contains all the fixes found in Exchange 2010 SP1 Update Rollup 1.