I would like to change the field type
It is not possible to change the field type of a field placed on the form.
For example, it is not possible to change a "Drop-down" field to a "Check box" field afterward.
If you want to use a field of a different type, place a new field on the form setting screen.
Using a CSV file, you can migrate data into the newly placed field.
For details on importing/exporting data, refer to the following pages.