Copy this to the "ThisWorkbook" code module and modify as necessary.
Private Sub Workbook_Open()
On Error Resume Next
Set myws = Nothing
Set myws = Worksheets("Sheet1") 'Modify
'Alternatively
Set myws = Sheet1 'Uses worksheet codename instead
On Error GoTo 0
If Not myws Is Nothing Then
myws.Range("A1").Value = myws.Range("A1").Value + 1 '<~~~modify
End If