If you are in the Focus Window, right-click on their box and click on Make File Root from the dropdown menu that appears.
If you are in any other window, select the Individual's record and then, on the Edit menu, click on File Root, and then Set as File Root.
You can change a file root at any time. You can also clear the file root (have no file root) if you wish. However, it is usually desirable to have a file root. The Focus Window always shows how the focus person is related to the file root, for example. The Records Window shows how everyone is related to the file root, in its Relationship to Root column.