Code in buttons layer, frame 1

target = 1;

btnPrev.onRelease = function(){
  target -= 10;
  if (target < 1){ 
    target = 1;
  } // end if
  _root.gotoAndPlay(target);
} // end btnPrev

btnNext.onRelease = function(){
  target += 10;
  if (target >= 40){ 
    target = 31;
  } // end if
  trace ("going to frame " + target);
  _root.gotoAndPlay(target);
} // end btnPrev

Code at end of each animation

_root.gotoAndPlay(_root._currentFrame - 9);