get the latest built from developers.google.com/android/nexus/images

unpack it to your android working dir. 
cat flash-all.sh for the current filenames (below is from mmb29q)

adb devices
adb reboot-bootloader
fastboot flash bootloader bootloader-deb-flo-04.05.img
fastboot reboot-bootloader
fastboot flash radio radio-deb-deb-z00_2.44.0_0213.img
fastboot reboot-bootloader
fastboot flash system system.img
fastboot reboot
 
all the data and settings survived and the device shows 
MMB29Q
1. Februar 2016 as Patchlevel.
 
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