Use a second MCU to read from the flash and program the Atmega328.
Check this for example - GitHub - MrBlinky/Arduboy-FX-mod-chip: Self updating bootloader for Arduboy FX
Use a second MCU to read from the flash and program the Atmega328.
Check this for example - GitHub - MrBlinky/Arduboy-FX-mod-chip: Self updating bootloader for Arduboy FX