Hey there! Unfortunately, it looks like you're running into a bug in our system. At the moment, you can't edit certain types of audience fields, such as dropdown, checkbox, radio, or select-multi member fields. This is an issue that we're aware of. I've made a note that you've run into the bug, and I'll notify you here when it's fixed.
In the meantime, the workaround is to create a new field that includes the dropdown options you want to use, and export and reimport your audience so the audience information goes into the new field. You're then free to delete the old field.
I hope this helps! Please let me know if you have any more questions!