To delete a tag definition:
From the Tasks section of the navigation bar choose Tags to open the Tags screen.
Click the tag you want to delete in the left panel.
If the list of tags is very long, use the Filter field to filter the list of tags. Enter part or all of a tag name to filter the list of tags. As you type the list is filtered, and lists only tag names that match your entered text.
Click Yes to confirm the deletion.