You can do this by modifying the "Items" property of DropDown.
For example, the following code within a CheckBox's callback will change the options you have in the DropDown object:
>> value = app.CheckBox.Value;
>> if value == 1
>> app.DropDown.Items = {'Option 3', 'Option 4'};
>> else
>> app.DropDown.Items = {'Option 1', 'Option 2'};
>> end
