volume-up and power button
(edited for summary: after installation device did not reboot – also pressing power button (alone) did not make it reboot – tried sudo fastboot reboot from linux, (w/o help) – then resorted to internet searches – 3rd link (and a bit of scrolling) lead me to the info, that pressing volume-up and power button forcefully reboots the device)
–
now up & running 4.5.0.16