Edit: Update Rollup 17 is now available for Exchange 2010 SP3 see the post Exchange 2010 SP3 Update Rollup 17 installation tips.
Run the Exchange2010-KB2685289-x64-en.msp file downloaded from the link above and click next at the welcome screen.
Accept the license agreement and click next.
Wait for the .Net assemblies native images to be generated. Then click next.
Exchange Services will now be stopped, files validated and installed and services started again.
Click finish to complete the install.
On my first attempt at running this update, the update failed to complete. Just before the update generates .Net assemblies, a User Account Control (UAC) dialogue box popped up and on clicking yes to continue, the update appeared to continue for a while then stopped without modifying any files, and with no error message. On a hunch I set the UAC to its lowest setting, rebooted the server then ran the rollup again without any problems.