1 - Open }HierarchyProperties cube
To define a default member for a dimension you need to open the }HierarchyProperties cube, if you can't find the cube in the menu list, you should turn on the Show/Hide Control Objects setting:
2 - Add default member
Open a view of the }HierarchyProperties cube, to define a default member, you need to populate the defaultMember element of the }HierarchyProperties:
In the example above the default member for the Currency is Local. If the default member is not defined for a dimension, the default member will be either the top consolidation and if there is no consolidation, the default member will be the first element in the dimension.
3 - Update the MDX hierarchies without restarting TM1 server
After configuring or editing the named levels in the }HierarchyProperties control cube, use the RefreshMdxHierarchy function to update the MDX hierarchies in the TM1® server without requiring you to restart the server.
Create a new TM1 process and add RefreshMdxHierarchy(''); in the prologue and finally execute the process, it should take a few seconds.
More information about RefreshMDxHierarchy function, in this IBM article.