SOURCE

let app = new Vue({
  el: '#app',
  data: {
  	count: 0
  },
  methods: {
  	countUp: function() {
    	this.count ++ ;
    },
    countDown: function() {
    	this.count --;
    }
  }
});

<div id="app">
  <h1>{{ count }}</h1>
  <button class="button" @click="countUp">countUp</button>
  <button class="button" @click="countDown">countDown</button>
</div>