Deleting template definitions should be done with caution and is generally only required after a major site redesign. Remember that definitions can be used across subsites, so even though your main site may no longer use a particular template, it could be in use elsewhere.
To delete a template definition, right click on it and choose "Delete template".
If the template name includes the word "default" you will not be able to delete it. If, for some reason, you did need to delete the default template, you'll need to rename it first.
Templates in Use
If the template definition you are trying to delete is in use, you'll see a warning at the top of the work area when you select the delete option. You can use the "Show content using this template" option in the actions panel to display a list of all of the articles currently using the template.
If you proceed with the deletion of a template definition that is in use, iCM will prompt you to assign a new template to those articles. A list of all available templates appears in a drop-down list. All articles using the template you are about to delete will be reassigned to the template you pick here.
Note that reassigning templates in this way will cause articles to behave strangely if the original or new templates have text blocks, constraints or article extras that do not match up. It is far better to assign new templates form within the article editor so that iCM can alert you to any potential problems.