Click the Generalization Link button 
on the diagram toolbar.
Click on the use case source element.
Drag the link to the destination use case element.
Drop when the second element is highlighted.
To create a generalization link between two use case elements:
Click the Generalization Link button
on the diagram toolbar.
Click on the use case source element.
Drag the link to the destination use case element.
Drop when the second element is highlighted.
NB: A generalization link applies only between use case elements and between actors.
Related Topics:
Creating Links
Moving Links
Editing Links