The in-place upgrade from Bright 9.0 to 9.1 has not yet been released. We will make an announcement when it's available!
You can do what we refer to as a parallel upgrade, where you deploy a new head node, in parallel with your existing head node(s) - meaning that this new head node has an interface on both the external and internal networks. Then you deploy a fresh copy of Bright on that new (parallel) head node. You can "lock down" the DHCP servers on both the old and new head nodes so that each only responds to the nodes it is managing.