<div class="col-md-6">Ich habe className col-md-6</div>
<div class="col-md-6">Ich habe className col-md-6</div>
<div class="col-md-6">Ich habe className col-md-6</div>
<div class="col-md-6">Ich habe className col-md-6</div>
<div class="col-md-6">Ich habe className col-md-6</div>
<div class="col-md-6">Ich habe className col-md-6</div>
<button>Ändere Classname</button>

<script>
  document.getElementsByTagName('button')[0].addEventListener('click',function(){
          var plusLinks = document.querySelectorAll('.col-md-6');
         plusLinks.forEach(function(g){
         g.setAttribute('class', 'col-md-4');
          g.innerHTML='Jetzt habe ich ClassName '+g.getAttribute('class');
          })

  })
</script>