Decreasing a field

[salmawy]

hi all am using vs2010
here is my connection string

Dim MySQLConnection As MySqlConnection
        MySqlConnection = New MySqlConnection
        MySQLConnection.ConnectionString = "server=db4free.net;Port=3306; User ID=seifhatem; password=11; database=huaweiunlocking"
        MySqlConnection.Open()

        Dim MyAdapter As New MySqlDataAdapter
        Dim SqlQuary = "SELECT * From Users WHERE Username='" & usernametextbox.Text & "' AND password = '" & passwordtextbox.Text & "';"
        Dim Command As New MySqlCommand
        Command.Connection = MySqlConnection
        Command.CommandText = SqlQuary
        MyAdapter.SelectCommand = Command
        Dim Mydata As MySqlDataReader
        Mydata = Command.ExecuteReader
        If Mydata.HasRows = 0 Then
            MsgBox("Error During Login:Please Enter Valid Data")
        Else

        End If

I want a command that decreases the field called Credits in the database and i will write between else and end if
like if the user clicks the button it connects and decrease 1 credit thx in advance