To link units from different courses so that any change made to a unit also applies to the linked one, go to the course's "Dashboard", click on the "Add Content" button and pick the "Clone unit" option. Select the course, then the lesson and, finally, the unit you want to use and check the "Retain relationship with original content (link)" (1) option.
Then, click on one of the two "Clone" (2) buttons for the selected unit to be added to the current course. In the future, any changes to the cloned unit will also apply to every other unit that is linked to the same source.
Note: See this article for more on how to edit a cloned unit without affecting the source unit.