<ul>
<li class="nav-top__list-item liborder"><a href="#" class="nav-top__link mbold">Пункт1</a></li>
<li class="nav-top__list-item ntides liborder"><a href="#" class="mshow" id="changeText">Пункт2</a></li>
<li class="nav-top__list-item"><a href="#" class="nav-top__link" target="0">Пункт3</a></li>
<li class="nav-top__list-item"><a href="#" class="nav-top__link">Пункт4</a></li>
</ul>

.mshow {
  opacity:1;
  transition: opacity 400ms ease-in-out ;
}
.mhide {
  transition: opacity 400ms ease-in-out;
  opacity:0;
}

var text = ["Замена слова раз", "Замена слова два"];
                var counter = 0;
                var elem = document.getElementById("changeText");
                setInterval(change, 3000);

                function change() {
                  elem.classList.add('mhide');
                  setTimeout(function(){
                   elem.innerHTML = text[counter];
                   elem.classList.remove('mhide');
                  }, 400);
                  
                  counter++;
                  if (counter >= text.length) {
                  counter = 0;
                  }
                }