Log in System : Change Password

I already have a code for Logging in and Registration. Can someone help me for Change password code? I think i just need to modify the code for log in. The only problem is I don't know how because the video tutorial that I've watched only covers the registration and log in. Thanks in advance

Imports System
Imports System.Data
Imports System.Data.SqlClient

Public Class HR_DatabaseManager

    Private Const CONNECTION_STRING As String = "Data Source=DAIMEGAERA\SQLEXPRESS;Initial Catalog=HRdepartment;Integrated Security=True"
    Private connection As SqlConnection = Nothing
    Private usersDataAdapater As SqlDataAdapter = Nothing
    Sub New()
        connection = New SqlConnection(CONNECTION_STRING)
        usersDataAdapater = New SqlDataAdapter("select * from Users", connection)
    End Sub
    Public Sub register(ByVal username As String, ByVal password As String)
        connection.Open()
        Dim usersDataSet As New DataSet()
        usersDataAdapater.FillSchema(usersDataSet, SchemaType.Source, "Users")
        Dim table As DataTable = usersDataSet.Tables("Users")
        Dim newRecord As DataRow = table.NewRow()
        newRecord("Username") = username
        newRecord("Password") = password
        table.Rows.Add(newRecord)
        Dim command As New SqlCommandBuilder(usersDataAdapater)
        usersDataAdapater.Update(usersDataSet, "Users")
        usersDataSet.Dispose()
        connection.Close()
    End Sub

    Public Function UsernameAvailable(ByVal username As String) As Boolean
        connection.Open()
        Dim usersDataSet As New DataSet()
        usersDataAdapater.FillSchema(usersDataSet, SchemaType.Source, "Users")
        usersDataAdapater.Fill(usersDataSet, "Users")
        Dim table As DataTable = usersDataSet.Tables("Users")

        For i As Integer = 0 To table.Rows.Count - 1
            Dim currentUser As String = table.Rows(i)("Username").ToString().Trim()
            If (currentUser = username) Then
                usersDataSet.Dispose()
                connection.Close()
                Return False
            End If

        Next
        usersDataSet.Dispose()
        connection.Close()
        Return True
    End Function
    Public Function Login(ByVal username As String, ByVal password As String)
        connection.Open()
        Dim usersDataSet As New DataSet()
        usersDataAdapater.FillSchema(usersDataSet, SchemaType.Source, "Users")
        usersDataAdapater.Fill(usersDataSet, "Users")
        Dim table As DataTable = usersDataSet.Tables("Users")

        For i As Integer = 0 To table.Rows.Count - 1
            Dim currentUser As String = table.Rows(i)("Username").ToString().Trim()
            Dim currentPassword As String = table.Rows(i)("Password").ToString().Trim()
            If (currentUser = username AndAlso currentPassword = password) Then
                usersDataSet.Dispose()
                connection.Close()
                Return True
            End If

        Next
        usersDataSet.Dispose()
        connection.Close()
        Return False

    End Function