...
You can mark your form private until you are done updating it, which will prevent new users from accessing the form, but if users happen to be completing the form when you switch it from public to private, they will see error messages. A better approach if feasible is to edit the form in a copy of your application running on a staging server. You can then replace the current form with the new form by removing the original application/form/flow from the staging server and uploading the new application/form/flow.