Two ways....
1) right click on the pivot, click pivot table wizard, hit the back
button until you get to the pivot table source.

2) if you're familiar with macros, you could try the code below. This
code will allow you to change the sources of multiple pivots at once.
I.e. if you change the source of 10 pivots each month to the same data
file, this code will automate the process.

Sub Update_Pivot_Table_Sources()
'
' Macro4 Macro
' Macro recorded 5/31/2007 by TB
'
'
   Dim iSheets As Integer, x As Integer
   Dim iPivot As Integer, _
       strCurrentSheet As String, _
       strNewPivotTblSrc As String, _
       strResponse As String
   Dim pt As PivotTable

strResponse = MsgBox("Do you want to change all of the Pivot Table
Sources?", vbOKCancel)

If strResponse <> vbOK Then MsgBox ("Cancelled")

If strResponse = vbOK Then

'see if the user selected a pivot table. if so, assign it to pt and
get the source
On Error GoTo NoPivotSelected
Set pt = ActiveCell.PivotTable
CurPivotTblSrc = pt.SourceData
GoTo Found_Pivot_Source

'if the user didn't select a pivot, see if there is one on the active
sheet.
'If so, use that. If not, return an error and exit
NoPivotSelected:
Resume RightHere
RightHere:
   On Error GoTo Error_Need_Pivot_Source
   CurPivotTblSrc = ActiveSheet.PivotTables(1).SourceData

Found_Pivot_Source:

strNewPivotTblSrc = InputBox("Please enter the new source for the
pivot table below", "New Pivot Source", CurPivotTblSrc)

   If strNewPivotTblSrc = "" Then
       MsgBox ("Cancelled")
       GoTo Exit_Update_All_Pivots
  End If

strResponse = MsgBox("Do you want to update all pivots (click Yes) or
just pivot tables with this data source: " _
                    & CurPivotTableSrc & " (click no)", vbYesNo)

   On Error GoTo Error_Found

   Application.ScreenUpdating = False

   'Count number of sheets in workbook
   iSheets = ActiveWorkbook.Sheets.Count

   'remember current sheet
   strCurrentSheet = ActiveSheet.Name

   If Windows.Count = 0 Then _
       GoTo Exit_Update_All_Pivots

   For x = 1 To iSheets

       'go to a worksheet to change pivot tables
       Sheets(x).Activate

       'turn warning messages off
       Application.DisplayAlerts = False

       'change all pivot tables on
       'this worksheet one at a time
       For Each pt In ActiveSheet.PivotTables

           If strResponse = vbNo Then
               If pt.SourceData = CurPivotTblSrc Then
                       pt.SourceData = strNewPivotTblSrc
                       ActiveWorkbook.ShowPivotTableFieldList = False
               End If
           Else
               pt.SourceData = strNewPivotTblSrc
               ActiveWorkbook.ShowPivotTableFieldList = False
           End If
       Next

       'turn warning messages on
       Application.DisplayAlerts = True
   Next

   'return to worksheet that you were originally at
   Application.ActiveWorkbook.Sheets(strCurrentSheet).Activate

   MsgBox ("Pivots updated successfully")

End If

GoTo Exit_Update_All_Pivots

Error_Found:
   MsgBox ("Error Found. Macro ending.     " & Err & ": " &
Error(Err))
   GoTo Exit_Update_All_Pivots

Error_Need_Pivot_Source:
   MsgBox ("Cannot find pivot table. Please select a sheet with a
pivot and re-run macro")
   GoTo Exit_Update_All_Pivots

Exit_Update_All_Pivots:
   Application.CommandBars("PivotTable").Visible = False
   Application.DisplayAlerts = True
   Application.ScreenUpdating = True

End Sub