Convert this INSERT statement into an UPDATE statement:
INSERT INTO myTable (day1,day2,day3......day30,day31) VALUES ('" .
$_POST["day1"] ."', '" . $_POST["day2"] ."', '" . $_POST["day3"] ."', ......
'" . $_POST["day30"] ."','" . $_POST["day31"] ."')
The ...... represent the respective days 4 throught 29.