To update bootloader / kernel / devicetree in a running system you have to copy the images in your tftp directory and upload them to the eMMC or SD Card.

Please see How to setup TFTP Server

1. Boot from eMMC or SD and stop autoboot in U-Boot

Hit any key to stop autoboot:  0
=>

2. Set U-Boot variable mmcdev according to the drive you want to write to (mmcdev 0 = eMMC, mmcdev 1 = SD card)
3. Connect device via ethernet to a tftp server supplying the image
4. Provide the correct network configuration in U-Boot:

• setenv autoload no
• setenv serverip <serverip> (e.g.: setenv serverip 192.168.100.110)
• setenv ipaddr <ipaddr> (e.g.: setenv ipaddr 192.168.100.111)
• setenv netmask <netmask> (e.g.: setenv netmask 255.255.255.0)

5. Provide U-Boot the file name of the desired image on the tftp server:

• For U-Boot:setenv uboot <filename>
• For device tree:setenv fdt_file <filename>
• For Kernel:setenv kernel <filename>

6. Perform Update:

• For U-Boot: run update_uboot
• For devicetree: run update_fdt
• For Kernel: run update_kernel