import { mapState } from "vuex";

computed: {
    ...mapState(["settings"]),
    computedProperty: {
      get() {
        return this.settings.valueInState;
      },
      set(valuePassedThrough) { //the value is passed through the v-model automatically
        this.$store.dispatch(`storeAction`, valuePassedThrough);
      }
    }
}

// ...
computed: {
  message: {
    get () {
      return this.$store.state.obj.message
    },
    set (value) {
      this.$store.commit('updateMessage', value)
    }
  }
}

<input v-model="message" placeholder="edit me">
<p>Message is: {{ message }}</p>