This changes the serial number from HIDFG to 0123456789. The file is USBCore.cpp.
#ifdef PLUGGABLE_USB_ENABLED
#if 0
char name[ISERIAL_MAX_LEN];
PluggableUSB().getShortName(name);
return USB_SendStringDescriptor((uint8_t*)name, strlen(name), 0);
#else
return USB_SendStringDescriptor((uint8_t*)"0123456789", 10, 0);
#endif
#endif