This URL is for the 2GB model, but is basically the same, and is the gist of flashing:
https://forums.developer.nvidia.com/t/jetson-nano-2gb-wont-boot-beyond-nvidia-splash-screen/220018/5
The part to keep in mind when flashing is that dev kits which use the SD card are slightly different than eMMC models (flash on an eMMC puts in everything, but an SD card model may have steps separate for the SD card versus the module itself).