Well, I finally figured out how to do this now. Ty for your reply!
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Excuse me @bilibili2011? Sorry to ping you but I would like to change the serial number for DSi NAND. Can you make a NAND Editor? I was confused under https://problemkaputt.de/gbatek-dsi-sd-mmc-firmware-misc-files.htm...
I tried to change MAC address on DSi firmware using hex editor, but Connection 1, 2, 3, and DS communications doesn't work anymore when I have changed the MAC address. Any way to change the MAC address without breaking such things?