In order to update an entire row (sorry, record) in a table you must use a subselect that returns enough values to fill every field.
UPDATE MyFileA SET ROW = (SELECT * From MyFileB WHERE MyField = “MyValue”)
This SQL statement will update every field in the MyFileA record with the values returned from the MyFileB select statement.
…or you can explicitly nominate the field/column (s) you want to update in the row.
e.g. UPDATE MyFileA SET MyFieldA = [MyFieldA Value], MyFieldB = [MyFieldB Value]…WHERE [Conditional]