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.

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