You must update a template that is currently Active and being used.

Export the existing template. Make changes to the XML files within the exported archive. When you reimport the archi automatically be updated.

In the existing template, create a new version and set an expiration date in the future. Create the new template and pt the same day.

Export the template and load it into the test site. Make changes on the test site template. When the template is ready to be published, choose "Publish to Production."

In the existing template, select the option to create a new version. When the template is ready to be published, publish the new version