Using FSO object to manipulate file names, this provides a template.  Note:
if file(s) are open or target name exists then you get an error, you need to
change code to improve.

Sub fName()
' set reference in vba project to Microsoft Scripting Runtime

Dim fso As New Scripting.FileSystemObject
Dim lFolder As Scripting.Folder
Dim lFile As Scripting.File

'set folder to act upon
Set lFolder = fso.GetFolder("C:\")

'loop through files
For Each lFile In lFolder.Files
If Left(lFile.Name, 5) = "00000" And Right(lFile.Name, 3) = "xls" Then
       lFile.Name = "12435" & Mid(lFile.Name, 6, Len(lFile.Name) - 5)
 End If
Next lFile

End Sub

Give it try.

Sub renamefile()
   Dim fs, f, fc, f1
   Set fs = CreateObject("Scripting.FileSystemObject")
   Set f = fs.GetFolder("C:\Cars") '<<==Change if not
   Set fc = f.Files
   On Error GoTo ex
   For Each f1 In fc
       f1.Name = "12345" & Mid(f1.Name, InStr(f1.Name, "_"))
   Next
   Exit Sub
ex:
   MsgBox "Can't change file name: " & f1.Name
   Resume Next
End Sub

keizi

Sub RenameFiles()
Dim sFileName As String
Const sStartDir As String = "C:\Cars"
Const sLookFor as String = "00000"
Const sReplace As String = "12345"

   sFileName = Dir(sStartDir & "\*.xls")

   Do Until sFileName = ""
       If Left(sFileName, 5) = sLookFor Then
           Name sStartDir & "\" & sFileName As Replace( _
               sStartDir & "\" & sFileName, "00000", sReplace)
       End If
       sFileName = Dir
   Loop

End Sub