How To Go Back To Previous Step In Wizard Control

I'm using the Wizard control to collect information about the users:

- Step1: User personal information
- step2: User account information (email. username, password ...)
- step3: User agreement
- step4: display all the inputted information before submitting

On Step2, after the user has inputted username and clicked the Next button (to go to Step3), I verify whether the username is already being used. If so, I want to display an error message and stop to process to go to Step3.

Here is the code that I'm using:

Code:
Protected Sub Wizard1_NextButtonClick(sender As Object, e As System.Web.UI.WebControls.WizardNavigationEventArgs) Handles Wizard1.NextButtonClick
Dim WhichStepId As String = Wizard1.ActiveStep.ID 'Just to see the active step
Dim WhatActiveStepIndex As Integer = Wizard1.ActiveStepIndex 'Just to know active step index

[Code]..