Controlling DC motor speed?
So first thing, you should try to be more disciplined with your code formatting and the question...

FYI: it's best to use #define as it doesn't use memory for storing pin numbers... It's like Find and Replace in a text editor, this:

const int controlPin2 = 4; // connected to pin 2 on the H-bridge
const int enablePin = 3;   // connected to pin 1 on the H-bridge


Becomes this:

#define controlPin2 4  // connected to pin 2 on the H-bridge
#define enablePin   3  // connected to pin 1 on the H-bridge


So this is the tidied up code, you can see the problem is a lot more approachable when the code is indented consistently, and good use of spaces and new lines is used...

#define controlPin2 4  // connected to pin 2 on the H-bridge
#define enablePin   3  // connected to pin 1 on the H-bridge


void setup () {
    pinMode( controlPin2, OUTPUT );
    pinMode( enablePin, OUTPUT );

    digitalWrite( enablePin, LOW );
    digitalWrite( controlPin2, LOW );

    Serial.begin( 9600 );
    Serial.println( "Ready" );
}

void loop() {
    if ( Serial.available() &gt; 0) {
        int inByte = Serial.read();

        switch ( inByte ) {

            case '0':
                digitalWrite( enablePin, LOW );
                digitalWrite( controlPin2, LOW );
                Serial.println( "OFF" );
            break;

            case '1':
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 50 );
                Serial.println( "CW slow" );
            break;

            case '2':
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 100 );
                Serial.println( "CW slow +" );
            break;

            case '3':   
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 150 );
                Serial.println( "CW medium" );
            break;

            case '4':   
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 200 );
                Serial.println( "CW fast" );
            break;

            case '5':   
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 250 );
                Serial.println( "CW fast +" );
            break;

            case '6':   
                digitalWrite( controlPin2, LOW );
                analogWrite( enablePin, 50 );
                Serial.println( "CCW slow" );
            break;

            case '7':   
                digitalWrite( controlPin2, LOW );
                analogWrite( enablePin, 100 );
                Serial.println( "CCW slow +" );
            break;

            case '8':   
                digitalWrite( controlPin2, LOW );
                analogWrite( enablePin, 150 );
                Serial.println( "CCW medium" );
            break;

            case '9':   
                digitalWrite( controlPin2, LOW );
                analogWrite( enablePin, 250 );
                Serial.println( "CCW fast" );
            break;

        }
    }
}


So your actual problem is that you need to reverse cases 1, 2, 3, 4, 5 to be 5, 4, 3, 2, 1...

There are two solutions:


Inelegant: would be to simply change the case numbers, they don't have to be in-order.
Elegant: copy and paste the code in each of the 5 cases about.


The result of option 2 is as follows, please feel free to confirm this is working?

#define controlPin2 4  // connected to pin 2 on the H-bridge
#define enablePin   3  // connected to pin 1 on the H-bridge


void setup () {
    pinMode( controlPin2, OUTPUT );
    pinMode( enablePin, OUTPUT );

    digitalWrite( enablePin, LOW );
    digitalWrite( controlPin2, LOW );

    Serial.begin( 9600 );
    Serial.println( "Ready" );
}

void loop() {
    if ( Serial.available() &gt; 0) {
        int inByte = Serial.read();

        switch ( inByte ) {

            case '0':
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 250 );
                Serial.println( "CW fast +" );
            break;

            case '1':
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 200 );
                Serial.println( "CW fast" );
            break;

            case '2':
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 150 );
                Serial.println( "CW medium" );
            break;

            case '3':
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 100 );
                Serial.println( "CW slow +" );
            break;

            case '4':
                digitalWrite( controlPin2, HIGH );
                analogWrite( enablePin, 50 );
                Serial.println( "CW slow" );
            break;

            case '5':
                digitalWrite( enablePin, LOW );
                digitalWrite( controlPin2, LOW );
                Serial.println( "OFF" );
            break;

            case '6':
                digitalWrite( controlPin2, LOW );
                analogWrite( enablePin, 50 );
                Serial.println( "CCW slow" );
            break;

            case '7':
                digitalWrite( controlPin2, LOW );
                analogWrite( enablePin, 100 );
                Serial.println( "CCW slow +" );
            break;

            case '8':
                digitalWrite( controlPin2, LOW );
                analogWrite( enablePin, 150 );
                Serial.println( "CCW medium" );
            break;

            case '9':
                digitalWrite( controlPin2, LOW );
                analogWrite( enablePin, 250 );
                Serial.println( "CCW fast" );
            break;

        }
    }
}