You can follow any of generally accepted upgrade options available:
- Using USB method explained in the How To
- CLI
sysupgradeor LuCI > System > Backup/Flash Firmware using thesysupgrade.itbimage - Owut, ASU, or the Firmware-Selector
- Booting in to the NAND recovery
initramfsand flashing thesysupgrade.itbfrom there.
Can you expand on your first two paragraphs? A simple diagram explaining your network devices and how you want them connected would help.