From the original rampsplus firmware you need to edit the pins.h file, motherboard 33 section and replace the relavent section with this. I allso attached the whole pins.h file if you want to just replace the whole thing
#if MOTHERBOARD == 33
#define KNOWN_BOARD 1
#define RAMPS_V_1_3
#elif MOTHERBOARD == 34
#define KNOWN_BOARD 1
#define RAMPS_V_1_3
#define AZTEEG_X3
#elif MOTHERBOARD == 35
#define KNOWN_BOARD 1
#define RAMPS_V_1_3
#define AZTEEG_X3_PRO
#endif
#if MOTHERBOARD == 3 MOTHERBOARD == 33 MOTHERBOARD == 34 MOTHERBOARD == 35
#define KNOWN_BOARD 1
#if !(defined (AVR_ATmega1280 ) || defined (AVR_ATmega2560 ))
#error Oops! Make sure you have 'Arduino Mega' selected from the 'Tools -> Boards' menu.
#endif
// uncomment one of the following lines for RAMPS v1.3 or v1.0, comment both for v1.2 or 1.1
// #define RAMPS_V_1_3
// #define RAMPS_V_1_0
#ifdef RAMPS_V_1_3
#define ORIG_X_STEP_PIN 54
#define ORIG_X_DIR_PIN 55
#define ORIG_X_ENABLE_PIN 38
#define ORIG_X_MIN_PIN 3
#define ORIG_X_MAX_PIN 2
#define ORIG_Y_STEP_PIN 60
#define ORIG_Y_DIR_PIN 61
#define ORIG_Y_ENABLE_PIN 56
#define ORIG_Y_MIN_PIN 14
#define ORIG_Y_MAX_PIN 15
#define ORIG_Z_STEP_PIN 46
#define ORIG_Z_DIR_PIN 48
#define ORIG_Z_ENABLE_PIN 62
#define ORIG_Z_MIN_PIN 18
#define ORIG_Z_MAX_PIN 19
#define ORIG_E0_STEP_PIN 26
#define ORIG_E0_DIR_PIN 28
#define ORIG_E0_ENABLE_PIN 24
#define ORIG_E1_STEP_PIN 36
#define ORIG_E1_DIR_PIN 34
#define ORIG_E1_ENABLE_PIN 30
#define ORIG_E2_STEP_PIN 42
#define ORIG_E2_DIR_PIN 40
#define ORIG_E2_ENABLE_PIN 44
#define SDPOWER -1
#define SDSS 53
#define ORIG_SDCARDDETECT 49
#define LED_PIN 13
#define ORIG_FAN_PIN 9
#define ORIG_PS_ON_PIN 12
#define HEATER_0_PIN 10
#define HEATER_1_PIN 8
#define HEATER_2_PIN 7
#define HEATER_3_PIN 45
// ANALOG NUMBERING
#define TEMP_0_PIN 13
#define TEMP_1_PIN 14
#define TEMP_2_PIN 15
#define TEMP_3_PIN 12
#define E0_PINS ORIG_E0_STEP_PIN,ORIG_E0_DIR_PIN,ORIG_E0_ENABLE_PIN,
#define E1_PINS ORIG_E1_STEP_PIN,ORIG_E1_DIR_PIN,ORIG_E1_ENABLE_PIN,
#define E2_PINS ORIG_E2_STEP_PIN,ORIG_E2_DIR_PIN,ORIG_E2_ENABLE_PIN,
#else // RAMPS_V_1_1 or RAMPS_V_1_2 as default