You will need to add a form with fields corresponding to the data you want to modify. Then, when the user submits it, you validate it (make sure it's safe and well formatted), and update the database with an UPDATE statement.
For a drop down menu (I'm assuming you mean a <select> box), you'll need to have a result set, iterate over it (while is the easiest), and create an <option> with the appropriate values on each iteration.