If for example your html code contain this code:
<select id="selectId"><option>Test1</option><option>Test2</option></select>
In order to change the list of option inside your select, you can use this code bellow. when your name select named selectId.
var option = $('<option></option>').attr("value", "option value").text("Text");
$("#selectId").html(option);
in this example above i change the old list of option by only one new option.