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

I have a mini app coded in VB.net. 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
        System.Diagnostics.Process.Start("C:/Application/dashboard.pbix")
    End Sub

    Private Sub OpenTracker_Click(sender As Object, e As EventArgs) Handles OpenTracker.Click
        System.Diagnostics.Process.Start("C:/Application/tracker.xlsx")
    End Sub

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



    End Sub
End Class