Updating sp2 to sp3
If your organization uses a load-balanced array of Client Access servers, see the following guidance about how to upgrade your Client Access servers from one version of Exchange 2010 to another, for example, from Exchange 2010 RTM to Exchange 2010 SP1.If you have an array of Client Access servers that you need to upgrade from one version to another, use the following steps.
I have tried twice to update to SP3 and both times it has failed.
I have tried via the windows update method and also downloading SP3 from your site and installing it.
After you upgrade Exchange 2010 to a newer service pack, you can't uninstall the service pack to revert to the previous version.
If you uninstall the service pack, you remove Exchange from the server.
I needed to document the process for a partner anyway so I might as well throw on here as well. The Exchange Team announced Exchange 2010 SP2 RTM on their blog recently.