If memory serves it is like this:
Code:
UPDATE   meetingbroker.dbo.masteruser MU
SET fieldname = HMU.fieldname
...

FROM     histmasteruser HMU 
WHERE    Mu.MasterUserID = HMU.MasterUserId AND MU.LastModifiedDate < HMU.LastModifiedDate
..but I have a sneaking suspicion that you can't use an alias for the MU table, so need to spell it out each time.