Does this mean that you want to change the worksheet that has a codename of
Sheet1 or do you want to change the leftmost worksheet in the workbook?

If it's the leftmost worksheet, you can use:
worksheets(1).Range("B10").formula = "=IF(E5=0,0,E5/C10/C13)"

If it's the codename, you could use:

Option Explicit
Sub changeFormulas()
   Dim WB As Workbook
   Dim fs As FileSearch
   Dim i As Long
   Dim Wks As Worksheet
   Dim fName As String

   Application.DisplayAlerts = False
   Application.ScreenUpdating = False

   Set fs = Application.FileSearch

   With fs
       ' change this to the folder you want
       .LookIn = "C:\Documents and Settings\JLClyde\Desktop\Steve"
       .LookIn = "C:\my documents\excel\test"
       .Filename = "*.xls"
       ' checks to see if there are excel files in the folder specified
       If .Execute > 0 Then
           For i = 1 To .FoundFiles.Count ' for each file found
               fName = .FoundFiles(i) 'sets workbook  name to be opened
               Set WB = Workbooks.Open(fName) 'opens the file
               With WB
                   Set Wks = Nothing
                   For Each Wks In WB.Worksheets
                       If LCase(Wks.CodeName) = LCase("sheet1") Then
                           Exit For
                       End If
                   Next Wks
                   
                   If Wks Is Nothing Then
                       MsgBox "no sheet with a codename Sheet1 in: " _
                                  & vbLf & WB.FullName
                   Else
                       Wks.Range("B10").Formula = "=IF(E5=0,0,E5/C10/C13)"
                   End If
                   
                   .Close savechanges:=Not (Wks Is Nothing)
               End With
           Next
       End If
   End With

   Application.ScreenUpdating = True
   Application.DisplayAlerts = True
End Sub