The minimum last step is to save your workbook and then quit.
myExcelWorkBook.Save()
myExcelApp.Quit()
It's important to quit Excel before the program ends. If you don't, an excel.exe process will continue to run on your computer until it's ended or your computer is rebooted, whichever comes first. When you're debugging, this can become a problem because you will probably terminate your program before ending the excel.exe process. In that case, you can end it in Task Manager.
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It should be noted that, while this VB.NET program will run fine, it doesn't clean up COM at all. One of the big advantages of .NET is that memory is reclaimed using "garbage collection". So when you Quit your program, .NET will figure out that the memory isn't needed anymore. COM, by contrast, relies on counts that are incremented and decremented as objects are instantiated and released again. COM deletes an object when the count reaches zero. In some cases (such as non-terminating server processes), you may have to use a ReleaseComObject method to decrement the reference count of a runtime callable wrapper (RCW) to keep things straight.
Source Code
Imports Excel = Microsoft.Office.Interop.Excel
Public Class Form1
Private Sub Button1_Click(...
Dim myExcelApp = New Excel.Application()
Dim myExcelWorkBook = myExcelApp.Workbooks.Open( _
"C:\ABExample.xlsx")
Dim myExcelSheet = _
myExcelWorkBook.Worksheets("ABWorkSheet")
myExcelSheet.Range("C5").Value += 54321.0
myExcelWorkBook.Save()
myExcelApp.Quit()
End Sub
End Class
