How can I update image?

I buy a ROCK960C. I run ubuntu on SD. How can I update image? There is nothing in the boot. I cannot replace old Image with new one.

I download rock960-model-C-ubuntu-server-arm64-20181011-1845-gpt.img. It works very well.
I download resource and recompile it and get a new Image. How can I replace the old Image with the new one. How can I get a new img file?

You do it the same way as you did the old one that you are replacing.

I can’t find the old Image. I have a img file. How can I find Image in the img file? How can reproduce a new img file?

What did you recompile exactly (the kernel?) and which steps/wiki did you follow?

I recompile kernel. cp rockchip_linux_defconfig .config ,make menuconfig , make. then I get new Image and new dtb

How can I reproduce a new system.img?

kernel is part of the boot image, you need to regenerate it: https://www.96boards.org/documentation/consumer/rock/build/linux-kernel.md.html

I download ext4 file . How can I reproduce a rootfs.img?

How can I reproduce a new img?