How to change a delivery to a template-Campaign?

Make sure the @isModel attribute for nms:delivery is set to yes. If not, update the field to yes either by Mass update or by using a workflow with query activity (select From nms:delivery where nmsdelivery_id=<PK>) and update data to change from no to yes.

Once done, go to the template by searching with Primary Key or Internal name. Right click the template and select action>Mass update of selected lines. Update the field @status to being edited from finished to make it usable again.