无法更新SQL数据库的数据 (新手问题)

小弟接触SQL没多久, 被这个问题折磨了很久. 希望坛友能解惑.不胜感激
其它都没问题,就是不能按我要的替换已有密码. (就是下面update那一段)
Imports System.Data
Imports System.Data.SqlClient

Public Class Form2
Dim con As SqlConnection
Dim adap As SqlDataAdapter
Dim cmd As SqlCommand
Dim ds As DataSet

Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    'TODO: This line of code loads data into the 'DatabaseStaffDataSet.Password' table. You can move, or remove it, as needed.
    Me.PasswordTableAdapter.Fill(Me.DatabaseStaffDataSet.Password)

    con = New SqlConnection("Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\DatabaseStaff.mdf;Integrated Security=True")
    con.Open()
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    adap = New SqlDataAdapter("SELECT * FROM [Password] where Username='" & Trim(ComboBox1.Text) & "'and Password = '" & Trim(TextBox1.Text) & "';", con)
    ds = New DataSet()
    adap.Fill(ds, "Password")
    Dim count = ds.Tables("Password").Rows.Count

    If count > 0 Then
        If Trim(TextBox2.Text) = Trim(TextBox3.Text) And Len(TextBox2.Text) > 2 Then
            cmd = New SqlCommand()
            cmd.CommandText = "update Password set Password= '" & Trim(ComboBox1.Text) & "' where Username = '" & Trim(ComboBox1.Text) & "'"
            cmd.Connection = con
            cmd.ExecuteNonQuery()

            MsgBox("Dear " & Trim(ComboBox1.Text) & ", Your password has been changed")

        ElseIf Trim(TextBox2.Text) <> Trim(TextBox3.Text) Then
            MsgBox("Your new passwords do not match", MsgBoxStyle.Critical)
        ElseIf Len(TextBox2.Text) < 3 Then
            MsgBox("New password must be at least 3 charactors", MsgBoxStyle.Critical)
        End If

    Else
        MsgBox("Current password Wrong, please check your usename and password", MsgBoxStyle.Critical)
    End If


End Sub

End Class

是基于VB.net的