marlin configuration.h
/**
* Select which serial port on the board will be used for communication with the host.
* This allows the connection of wireless adapters (for instance) to non-default port pins.
* Serial port 0 is always used by the Arduino bootloader regardless of this setting.
*
* :[0, 1, 2, 3, 4, 5, 6, 7]
*/
#define SERIAL_PORT 0
Seriously did you even try and look?...
/**
* Select which serial port on the board will be used for communication with the host.
* This allows the connection of wireless adapters (for instance) to non-default port pins.
* Serial port 0 is always used by the Arduino bootloader regardless of this setting.
*
* :[0, 1, 2, 3, 4, 5, 6, 7]
*/
#define SERIAL_PORT 0
Seriously did you even try and look?...