父组件：

<template>
    <div>
        <up :myMessage="value"  @update:myMessage="func"/>
    </div>
</template>

<script>
import up from './update'
export default {
    components: {
        up
    },
    data() {
        return {
            value: ''
        }
    },
    methods: {
        func(e){
            this.value = '父组件点击子组件并修改了子组件里面的值';
        }
    }
}
</script>

子组件：

<template>
<div>
    <button @click="func">点击我改变</button>
    <p>{{myMessage}}</p>
</div>
</template>

<script>
export default {
    name: "update",
    props: ['myMessage'],
    methods: {
        func(params) {
            this.$emit('update:myMessage',params);
        }
    }
}
</script>