My Deco M5 has been supplied to me by my service provider. (pack of 3). One of the routers conked off. I called up my ISP. They replaced it and connected through app... It was working. Now the replaced device has olden firmware. Suddenly my app asks me to upgrade to latest firmware and if i do, it finds the main router to be in the latest and does nothing. Then again it says my router needs to be upgraded.

Not sure how to upgrade the firmware of only one router in the mesh. (I do not want to replace this router as the main router)

Any idea/help will be appreciated