Templates, like other files, are deleted in the Finder, not in the application where a link to the template is displayed.
Go to the finder.
Hold down the option (alt) key as you go to the Go menu and choose Library.
This takes you to your User Library.
Double click each item in this path:
(Library) > Application Support > iWork > Numbers > Templates > My Templates
In the My Templates folder, you will see a list of your custom templates.
To move a template to the Trash, do one of these three actions:
- Click on the template's icon and drag it to the Trash. OR
- Right click on the template's icon, then choose Move to Trash from the contextual menu, OR
- Click on the template's icon to select it, then press command-delete.