On Desktop and Web
You can delete a tag by first selecting the tag on the left most panel, then pressing the Delete option that appears under the tag name.
If the Folders extension is active, you'll have to select the tag, then press the minus icon that appears beside the tag name.
On iOS and Android
Delete a tag by long-pressing the tag until the action menu appears, then tap on the Delete option. This works when selecting tags on the left-side menu on the notes list, as well as the right side menu on an open note.