You can find the details at the Microsoft Exchange blog here: 

http://blogs.technet.com/b/exchange/archive/2011/10/28/released-update-rollup-6-for-exchange-server-2010-sp1.aspx

The Microsoft Exchange Team blog has the details:

http://blogs.technet.com/b/exchange/archive/2011/09/22/released-update-rollup-5-for-exchange-server-2007-sp3.aspx

Sometimes you may forget the last Update Rollup you installed.  It's not necessarily intuitive as to how to figure this out on your own.  Here's a how-to from the Microsoft Exchange blog:

http://www.howexchangeworks.com/2011/09/finding-which-update-rollup-is.html

As of October 11, 2011, Exchange 2010 RTM (that is, without any Service Packs) is losing support from Microsoft.  Time to make plans for updating to the latest Service Pack and Update Rollup.

The announcement from the Microsoft Exchange blog can be found here:

http://www.howexchangeworks.com/2011/09/time-to-upgrade-to-exchange-2010-sp1.html