On the front side of the Arduboy there are 3 LEDs and 3 resistors in the bottom left corner right above the reset button.
Its a wild guess but I think they are MISO, MOSI, SCK (not sure in which order) You can use these pins if you find out the right order. For VCC I am not sure where to start.
SCKは見つかって赤い部分と分かったのですが、前半田付けした時に基板が焦げてしまってMOSIが見つけられないです
I could find the thru hole to SCK as red one but I couldn’t find the thru hole to MOSI because I burned the surface before I tried soldering
My one is also a product version. I hope the picture helps you finding the via.
When my one was bricked I could not solder as I run out of tips for my soldering iron. So I had to find another solution.
Maybe you can create some pads out of aluminium foil or steel wool. If there are some pieces left from the original pad there might be the chance the pad can connect to the leftovers of the lands so you can flash a bootloader.