I guess you would do it like the other updates with the Spark. ?
I've never used the DJI assistant, but you may have too since this is a new mother board install.
All of the times I've updated the Spark was through the Home Wi-Fi and the Spark Wi-Fi.
You have to download the packet from the Home Wi-Fi and then switch to the Spark Wi-Fi to install it when the app says "installing".