To delete a concept, click on the requested concept and select DELETE icon from the popup bar above it. (pic. 1)
Pic. 1
To delete the connecting line or both the connecting line and the name of the relationship, click on the name of the relationship or the green small button on the connecting line and select DELETE from the selection above.
To delete a name of the relationship:
activate the bubble with the name of the relationship and click again on the text and press DELETE or,
click on the name of the relationship and select RENAME and delete the text, than you may press enter or click DONE.