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Download the image file for Ubuntu Server from from http://cdimage.ubuntu.com/ubuntu/releases/18.04.3bionic/release/ubuntu-18.04.35-preinstalled-server-arm64+raspi3.img.xz, then insert your microSD card into your computer so that you can flash the disk image onto it. If you are running Ubuntu or any other Linux distro, you can flash the image using this command:
xzcat ubuntu-18.04.35-preinstalled-server-arm64+raspi3.img.xz | sudo dd bs=32M of=/dev/mmcblk0 iflag=fullblock oflag=direct status=progress; sync
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