As of right now, once you've saved a document to Sparrow as a template, you won't be able to add or remove placeholders from the document. If you want to modify the template, you will need to first delete it (from Sparrow, not the Google document), then re-create it.
Delete Sparrow Template
With the Sparrow add-on open in the template you'd like to change, click "Manage Template"
Next Click Delete Template and confirm.
Your template is now deleted in Sparrow.
After you refresh the add-on, you will be able to add and remove metrics just as you were previously able to do. Once you're ready to save it as a template again, you can click the Save as Template button to create a new Sparrow template.