1. If you want to add fields or edit fields to an existing template, first Clone the template. To clone a template go to Admin > Templates. Choose the template you want to clone and click "Clone". The cloned template will appear at the bottom of the list.
2. Make edits to the cloned template
3. Disable the original template and rename it. Eg. Template (Old version, Date). You do not have to change the template key of the old disabled template.
4. Rename the cloned template to the original name and Enable it.
5. iOS users will need to hit Refresh Configuration in their app to update the templates.