Copy the U-Boot debian package to your board and install it.

sudo dpkg -i linux-u-boot-vim4-vendor_xxxx-2019.01_arm64.deb
sync
sudo reboot

You need to setup Seria Debug Tool first, then you can directly upgrade U-Boot from the console.

The installed U-Boot is located at build/u-boot/fip/_tmp/u-boot.bin.signed.

• Power on the board and enter the U-Boot console
• Copy `u-boot.bin.signed` into an SD card or thumbdrive
• Insert the SD card or thumbdrive into the board
• Execute the following commands to upgrade U-Boot

kvim4# usb_update bootloader u-boot.bin.signed
kvim4# reboot

Build U-Boot.