Change hotend thermistor input in Marlin
In pins_RAMPS.h you find which pins are used for which thermistor:


//
// Temperature Sensors
//
#define TEMP_0_PIN         13   // Analog Input
#define TEMP_1_PIN         15   // Analog Input


To use the other thermistor, you would need to swap the numbers:


//
// Temperature Sensors
//
#define TEMP_0_PIN         15   // Analog Input
#define TEMP_1_PIN         13   // Analog Input