Case access privileges can be altered in two places:
1) The most common way is to go to a case's case dashboard and click the Access icon (bottom right of the case dashboard icon window) and change who can access the case there. This video shows the steps: Collaborating and Assigning Case Access Rights - YouTube
2) Administrators can also go to the Admin link at the top of their user dashboard. There, they have the option of reassigning ALL cases from one attorney to another.