Hey ,let me make this a bit clear People above have been saying that the upgrade is not possible, Itis not correct
Upgrade is possible, but it is NOT SUPPORTED.
This migration path is not supported.
RU6MP3 is a Maintenance patch and not a maintenance release, A MP can only be installed on the main build. You can install RU6 over RTM or MR3 or MR4MP2 but not RU6MP3.
For that you need to have RU6
It is not recommend to upgrade directly from RU5 or MR4 to RU6MP3
This migration path is not supported.
Possible effects after a direct upgrade to RU6MP3 from a lower version [except RU6]:
Schema Corruption.
Possible dll corruption.
Policies Corruption.
Network breakdown.
Challenges with uninstalling SEP.
So what you need Upgrade the clients to RU6 and then upgrade to RU6MP1.