Updating B1300s in a mesh

I now have three B1300s in a mesh and all is well. When a firmware update is released and I install will it go only to the main node router, or to all three At once? If only one, do I have to break the mesh, install to all, and rebuild the mesh?

They will all update at the same time.

@Top362nd
As scagney said, the firmware update will update the primary and secondary routes synchronously.