How to Update IMX6ULL partially

hello experts
i want to update u-boot or kernel only
not by using toradex eazy installer , i read about “Partial updates” using TFT/NFS Server
on Downloads & Installers | Toradex Developer Center

but this page doesn’t explain enough for me , i booted on u-boot and downloaded a new u-boot
how could i update u-boot on nand ?
i tried nand related command ( nand erase and nand wrire command) but failed
and there are two u-boot sectors which one shoud i update ?

thanks in advance


please refer to below article for this request.