Public Sub sost()
        TextBox1.Text.Replace("ciao", "buongiorno")
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If TextBox1.Text = "" Then
            MsgBox("Vuoto!", MsgBoxStyle.Information)
        Else
            sost()
        End If

    End Sub