If you're using MailChimp's Facebook Application and you have your signup forms on your Pages, you would make all edits to your signup form the usual way in MailChimp. You'll find your signup forms under MailChimp > Lists > Forms.
Once you make changes to your forms in MailChimp, we'll push them on over to Facebook. You can refresh your Facebook Page to see the changes pushed through to the Facebook MailChimp Application.