Program the firmware from U-Boot
Digi recommends you program your images into the internal NAND using the firmware update mechanism.
You can also use U-Boot to program your ConnectCore 6UL SBC Pro. If your ConnectCore 6UL SBC Pro is not booting U-Boot, you need to boot it from USB. See Boot from USB.
U-Boot fetches the images from a TFTP server in your Linux workstation and programs the images into the NAND. If you don't have a running TFTP server, follow the installation instructions in Install Digi Embedded Yocto.
You can program the firmware from either TFTP or microSD: