File content as of revision 2:b20570f160c6:

#include "mbed.h"
#include "MODSERIAL.h'

DigitalOut Direcion_M1(D4);
DigitalOut Speed_M1(D5);
//DigitalOut Speed_M2(D6);                  
//DigitalOut Direction_M2(D7);

InterruptIn Switch_1(D8);
int counter_extension=1;

MODSERIAL pc(USBTX, USBRX);  

void extension (){
    switch (counter_extension){
        case 1:
            digitalWrite(Direction_M1, 1);   //The arm will get longer  
            analogWrite(Speed_M1, 255);      //The motor is turned on
            pc.printf("The arm will now get longer");
            wait(0.5f);
            break;
        case 2:
            digitalWrite(Direction_M1, 1);   //The arm will get longer  
            analogWrite(Speed_M1, 0);        //The motor is turned off
            pc.printf("The arm will now stop");
            wait(0.5f);
            break;
        case 3:
            digitalWrite(Direction_M1, 0);   //The arm will get shorter  
            analogWrite(Speed_M1, 255);      //The motor is turned off
            pc.printf("The arm will now get shorter");
            wait(0.5f);
            break;
        case 4:
            digitalWrite(Direction_M1, 0);   //The arm will get shorter 
            analogWrite(Speed_M1, 0);        //The motor is turned off
            pc.printf("The arm will now stop");
            wait(0.5f);
            break;
    }     
}                 

void switch_counter_extension (){
    counter_extension++;
    if (counter_extension > 4){
        counter_extension=1;
    }
    extension();
}

int main(){
    pc.baud(115200);
    pc.printf("RESET \n");
    
    digitalWrite(Direction_M1, 1);   //The arm will initially get longer  
    analogWrite(Speed_M1, 0);        //The motor is initially turned off
    
    switch_1.rise(&switch_counter_extension);  
}