如何在不同vue方法内的函数内调用vue方法[duplicate]

methods: {
  updateUserToPremium(userId) {
    axios.post(`/user/${userId}`, {}, {
      headers: { "Authorization": `Bearer ${this.$store.state.token}` }
    }).then(res => {
        console.log('Success session creation');
        this.routeToStartpage()
    }).catch(error => {
        console.log(error);
        return error;
    })
  },

  testing(userId) {
    var rp = new Reepay.ModalSubscription(sessionId);
    rp.addEventHandler(Reepay.Event.Accept, function(data) {
        console.log('Success', data);

        updateUserToPremium(userId);  // <--- This triggers updateUserToPremium is not defined' 
    
        rp.destroy();
    })
  },
}