There is a lecture available:
I saw a Mendix webinar once where they recommended to build a MF that updates the records in batches of 500.
Can't seem to find the link to that webinar though...
If you have the authorisation: Run an UPDATE-query in the database. By far the fastest way.
But this is a Mendix forum, so you will want to build it using Mendix, go with lecture/85 like Aswin says.