Deleting a Derived Template is as simple as deleting any regular Info Item. Simply delete the Template Info Item or reparent it away from the system Templates Info Item (except for the Recycle Bin, which has special behaviour *).
When a template is permanently deleted (not to the Recycle Bin*), any Info Items previously based on this template will be re-assigned the first remaining ancestor template automatically by Ultra Recall.
For example, let's assume you create a template called Recipe, derived from the core Text template. You then create some Info Items based on this new Recipe template. You later decide you don't want the Recipe template and delete it.
If you permanently delete the Recipe template, all existing Info Items based on it will be automatically updated as being based on the Text template (the first existing ancestor template of the now removed Recipe template)