Edit in JSFiddle

var app5 = new Vue({
  el: '#app-5',
  data: {
    message: 'Merhaba Dünya'
  },
  methods: {
    reverseMessage: function () {
      this.message = this.message.split('').reverse().join('')
    }
  }
})

<script src="https://unpkg.com/vue/dist/vue.js"></script>
<div id="app-5">
  <p>{{ message }}</p>
  <button v-on:click="reverseMessage">Yazıyı Ters Çevir</button>
</div>