Some questions:

1) Is this all in one table or are multiple tables involved?

2) What version of Access are you using?

You should be aware that "" is looking for an empty string, while NULL is something entirely different, and you need to test for both conditions. It appears that the SQL syntax you are using is T-SQL (SQL Server) oriented, while Access uses a somewhat different syntax for some kinds of operations. I would suggest you try using a SELECT query to get the exact set of records that you want to update using the Access query builder, and then change it to an Update query when you are sure you are working with the correct record set.