You can make updates to custom Units that you have written. If your groups are currently using those Units, they will not see any updates you make until they make their next Course. 

When you create a Course, the web app takes a snapshot of each Module that you then add on to with your own comments and answers. Because of this, any changes to a Unit within a Module will only be seen the next time a course is created and saves a new snapshot of the current Units. 

No course is editable once it has been created.