1. Make a backup of your workbook.
2. Open your workbook and ALT+F11
3. Locate your Workbook name in Project Explorer Window
4. Double Click on "This Workbook"
5. Copy paste the Macro code given - Replace Sheet1 and B1 as per your need
6. Save your file as .xlsm if you intend to reuse Macro again.

< A workbook illustrating this can be downloaded from Invoice Number Generator >

'**** Macro Starts

Private Sub Workbook_Open()
Worksheets("Sheet1").Range("B1") = Worksheets("Sheet1").Range("B1") + 1
End Sub