Spoon Feeding you for the last time :)
- Declare your Objects/Variables. The working becomes much easier if you use them. Avoid the use of ActiveWorkbook/ActiveSheet/Selection..etc.. 
INTERESTING READ
- There is a difference between variables and strings. Use them appropriately. If you enclose variable names in "then they become string and vice versa i.e if the string is of one word.
- You are subtracting 1 from a string Format(Date, "YYYYMMDD") - 1.Formatreturns aString. See the alternative that I have given in the code below.
- While doing a Save Asin Excel VBA, you have to specifyFileFormatfor example, xlsm will have a fileformat of 53 below are the other file formats.
File Formats
50 = xlExcel12 (Excel Binary Workbook in 2007-2013 with or without macro's, xlsb)
51 = xlOpenXMLWorkbook (without macro's in 2007-2013, xlsx)
52 = xlOpenXMLWorkbookMacroEnabled (with or without macro's in 2007-2013, xlsm)
56 = xlExcel8 (97-2003 format in Excel 2007-2013, xls)
Your code (UNTESTED)
Sub openwb()
    Dim wb As Workbook
    Dim sPath As String, sFilename As String, newName As String
    sPath = "E:\sarath\PTMetrics\"
            sFilename = sPath & _
                "D8 L538-L550_16MY_Powertrain Metrics_" & _
                Format(Date - 1, "YYYYMMDD") & _
                ".xlsm"
    Set wb = Workbooks.Open(sFilename)
    newName = sPath & _
              "D8 L538-L550_16MY_Powertrain Metrics_" & _
              Format(Date, "YYYYMMDD") & _
               ".xlsm"
    wb.SaveAs newName, FileFormat:=53
    wb.Sheets("All Concerns").Range("A1").Value = "Hay..."
End Sub