Digi Embedded Yocto offers different ways to program the firmware in the device. If you don’t have any firmware flashed in your device or the one you have is corrupt and does not boot, you should program the firmware from U-Boot. You can also update the firmware from U-Boot if you want to use the build deliverables from a Digi Embedded Yocto project.
If you have a Yocto system already running you can update the firmware using a software update package. To learn more about how to generate an update package see Build a software update package.