I would recomment you to brake the code into smaller methods/functions

stage.addEventListener(KeyboardEvent.KEY_DOWN, myKeyDown);
function myKeyDown(e:KeyboardEvent):void{
 
    switch (e.keyCode){
        case Keyboard.LEFT :
            this.turnLeft();
            break;
        case Keyboard.RIGHT :
          this.turnRight();
            break;
    }
}
 
function turnLeft(){
            prevFrame();
            if (currentFrame < totalFrames){
                nav_mc.next_btn.mouseEnabled = true;
                nav_mc.next_btn.alpha = 1;
            }
            if (currentFrame == 1){
                nav_mc.back_btn.mouseEnabled = false;
                nav_mc.back_btn.alpha = .6;
                nav_mc.text_btn.mouseEnabled = false;
                nav_mc.text_btn.alpha = .6;
           }
}
 
function turnRight(){
            nextFrame();
            if (currentFrame > 1){
                nav_mc.back_btn.mouseEnabled = true;
                nav_mc.back_btn.alpha = 1;
                nav_mc.text_btn.mouseEnabled = true;
                nav_mc.text_btn.alpha = 1;
            }
            if (currentFrame == totalFrames){
                nav_mc.next_btn.mouseEnabled = false;
                nav_mc.next_btn.alpha = .6;
            }
}
 

Also, better to add states for buttons, like enabled() and disabled() methods

Back button code example:

function enable() {
     this.mouseEnabled=true;
     this.alpha=1;
}
function disable() {
     this.mouseEnabled=false;
     this.alpha=0;
}

Navigation panel code:

function enableBackButton() {
     this.back_btn.enable();}
function disableBackBtn() {
     this.back_btn.disable();
}
 
 
 
 

Later this will allow you to easier change UI elements behaviour, for example, when you will need to add animation for "enable" state. To make development more comfortable try to use classes, with classes you will be able to write code once and use it for all buttons or other UI elements in the movie.