Solved, here is how
Step1) set up serial http://docs.khadas.com/develop/SetupSerialTool/
Stem2) create a flashable sdcard for android http://docs.khadas.com/bootcamp/CreateBurnCardViaCLI/
Step3) insert sdcard
step4) erase emmc http://docs.khadas.com/develop/HowtoEraseEMMC/
after reboot, it will automatically start the upgrade process
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