Flashing BeagleBone Black image
Judging by the file name, you have downloaded a filesystem image. To use it, you need to take an empty SD card, at least 2 GB large, and extract your image on it:

xz -cd *.img.xz | dd of=/dev/sd_card_dev_name bs=8MB


Then you will need to remount the SD card you have extracted the image to, and edit /boot/uEnv.txt on it to enable flashing. Then put the SD in your BeagleBone and boot from it to actually flash the eMMC.

Note that I don't actually own a BeagleBone, so take is as advice, not as 100% working solution. And please comment back if you have anything to add.