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.
Here's how:
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.