How to close an excel file and a power BI file using application

I have a mini app coded in Its function is to open an excel and power bi file by clicking buttons on a windows form and close it after using it . I can successful open the file by clicking the buttons . I'm kindly requesting anyone to help me with a code that I can use to close the excel file and the power BI file by clicking on different buttons.
Here is my code for opening the files:
Public Class Form1

    Private Sub OpenDashboards_Click(sender As Object, e As EventArgs) Handles OpenDashboards.Click
    End Sub

    Private Sub OpenTracker_Click(sender As Object, e As EventArgs) Handles OpenTracker.Click
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Me.Visible = False

    End Sub
End Class
Douglas SeyamaSystems DeveloperAsked:
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May be you should open file by Excel, but not  " System.Diagnostics.Process.Start"
For example,
 Dim xlApp As Excel.Application
            Dim xlWorkBook As Excel.Workbook
            Dim xlWorkSheet As Excel.Worksheet

            xlApp = New Excel.ApplicationClass
            xlWorkBook = xlApp.Workbooks.Open(myFile)
            xlWorkSheet = xlWorkBook.Worksheets(1)

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if you want to close file whithout save

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