Maybe this will help...
Where do I paste the code that I find on the internet
You should also declare variables within your vba procedures.
Define Variables In VBA
Code:
Option Explicit
Sub test()
Dim Cell As Range
Dim ws As Worksheet
For Each ws In Worksheets(Array("130R", "135R", "140R"))
For Each Cell In ws.Range("E4:P45")
If Cell.Value > 0 Then
Cell.Value = Cell.Value * -1
End If
Next Cell
Next ws
End Sub