You can view and modify the list as needed. Plase keep in mind you will need to re-approve landing pages hosting the updated form.
Open the landing page in Design Studio and click the link to "Form".
In the form, click the Country field. The "field specific" property sheet displays "Select" for "Input Type".
The first entry of "Value List" must be a dummy vallue. It is not selectable. The second and beyond are valid for selection.
Values are separated by commas.
You can add or remove values anytime and then re-approve landing pages.