Update Table1 Set Field1 = 
(Select Field2 From Table2 Where Table1.KeyField = Table2.KeyField)

--HowTo - Update from another table using SubSelects

UPDATE sysdba.CONTACT
  SET ACCOUNT = (SELECT a.ACCOUNT 
                 FROM ACCOUNT a
                 WHERE CONTACT.ACCOUNT = a.ACCOUNTID)
WHERE ISNULL(CONTACTID, '') = '' AND
      USERFIELD10 <> '' AND
      EXISTS (SELECT * 
              FROM ACCOUNT a
              WHERE CONTACT.ACCOUNTID = a.ACCOUNTID)