I think you have two choices:
a) The simplest: reverses the direction of rotation of the X motor in the RAMPS board. It's as easy as unplugging the X motor and plugging again after having rotated 180 degrees.
b) The more Pro alternative: modify the Marlin firmware. Open it in the Arduino IDE and search the following lines at the config.h tab:
#define INVERT_X_DIR false //Try to set true
#define INVERT_Y_DIR true
#define INVERT_Z_DIR false
#define INVERT_E0_DIR false
#define INVERT_E1_DIR false
#define INVERT_E2_DIR false
// ENDSTOP SETTINGS:
// Sets direction of endstops when homing; 1=MAX, -1=MIN
#define X_HOME_DIR -1 //Try to set 1
#define Y_HOME_DIR -1
#define Z_HOME_DIR -1
