From the extension yes, then re-add with the new template and re-provision. Obviously you will need to load the template into the system (if using a custom template) and FYI custom templates are "un-supported" with 3CX.
Please note that if a phone is already provisioned using the default template you need to delete the phone from the extension and re-add it using the custom template. As eddv123 mentioned custom templates are not supported by 3CX but if you are still going to use one please have a look at the following guide on how to correctly create one https://www.3cx.com/docs/custom-ip-phone-templates/