Cannot Update A Record

What's the problem in the following code ?
How can I write an alternate?

VB Code:

sql = "SELECT A.EMP_DEPT_CODE FROM EMP_MASTER A, EMP_DEPT B WHERE A.EMP_ID = B.EMP_ID AND B.EMP_DEPT_ID = 3"
rst.open sql, DB_CONN, adOpenDynamic, adLockOptimistic
Do Until rst.EOF
  rst("EMP_DEPT_CODE") = "NEW_CODE"
  rst.Update
  rst.MoveNext
Loop
rst.Close

It's giving error

Run-time error '3251':
Current Recordset does not support updating. This may be a limitation of the Provider, or of the Selected locktype.

What is "NEW_CODE"?

I never use the .Upate Subroutine. I just use UPDATE SQL statements. Try that instead.