If you created a room and want to change it from private to open, or vice versa, you can do so by going to your mural settings.
Go to the room you'd like to modify.
Choose "Room settings" from the drop-down menu next to the room name.
From the drop-down menu under "Room type," choose the option you want.
*Note: Private rooms are available only to those who have been invited in; open rooms are available to everyone in the workspace.