How to update a table from a another table
An alternative approach to the co-related subquery (suggested by Kerri) would be to use the MERGE statement, which might be more efficient than the subselect (which can only be verified by looking at the execution plan of both statements). 

MERGE INTO table_b 
USING 
(
  SELECT id,
         field_2
  FROM table_a
) ta ON (ta.id = table_b.id)
WHEN MATCHED THEN UPDATE 
    SET table_b.field_2 = ta.field_2