To delete topics and elements, select it. Next:
Mac and Windows: use the Delete button.
iPad&iPhone and Android: use delete delete from the (overflow) menu. Or use the topic toolbar and cut Cut.

Deleting a topic deletes the whole branch. Mac and Windows: Delete a single topic via the the main menu - "edit" - "delete" - "delete single topic".
Deleting a cross-link or relation does not delete topics.