It sounds like you are trying to modify the values in the dropdown list. If this is it, here's some info that should help.
When you add a field as a dropdown type what you are actually doing is referencing a value in another table. So, in order to edit the text content of a value in this list, you'll need to use a screen that is built upon the referenced table. The good news is that if you added the vaules at the same you created the dropdown list, Method typically creates a screen for that table. Look in your list of screens for one named the same as the dropdown field you created.