: Open a terminal in the folder containing your downloaded boot.img and run: fastboot flash boot boot.img .
Flashing the wrong boot.img for a different ROM version will cause a bootloop. Ensure the image matches your exact crDroid build date/version. crdroid bootimg install
Once the device is in Fastboot mode (usually a black screen with the Android robot or text), verify your PC can see it: : Open a terminal in the folder containing