I would do this in a few steps (you can later go back and nest the code if you want to make it all one query).
1. Make a Select Query similar to what you have already, but add the following criteria under [CStat Data].DataDate:
2. Create a new a new query based on the first query, that is an Aggregate Query that returns only the latest (most recent value) of [CStat Data].DataDate (Group By your other field(s)).Code:<=[HLT Data].NewDate
3. Use this query in an Update Query to update your values.