Unfortunately an update to Sierra didn't work out. The computer froze during reboot. I managed however to successfully install a fresh version of Sierra by using a USB stick with unibeast. Right now I'm running happily 10.12.5 stable on my computer.
I recommend the AirPort Extreme hardware to everybody. In combination with time machine the restoration of my user files took only little time.
One point of remark: I have a tonymac recommended build from a few years ago. I had to install the network driver from the Yosemite Multibeast in order to get LAN network going. The Multibeast from Sierra does not contain the right driver anymore.