Is It Possible to Update Cells of Multiple Files?

hello again

the last time i was posting here, i had a question about updating headers in multiple worksheets. Teylyn offered me some great advice - at this point I'd like to say thanks again. I built up a lot on this code, and it really helps me every day.

I'm still working on multiple updates of various tables, sheets, cells, headers....
however, today i've come across a challenge which I am not able to pass alone

I'm trying to achieve the following:
I have 5 excel FILES in a folder.
what i would like to do now is to update a certain cell (or the header, doesnt matter..) in all 5 files. is this even possible with VBA?

starting with the code from the last post, i think the line with the activeworkbook-code is where the change has to be. the problem is that i don't know where to start

any help or advice would be very much appreciated!

Sub CellInHeader()
Dim ws As Worksheet
Dim hdr As String
Dim arrSheets
'set the array of sheets to be worked on
'list the sheet names here
arrSheets = Array("Sheet1", "Sheet2", "Sheet3")

'turn off screen updating to speed up the macro
Application.ScreenUpdating = False

For i = LBound(arrSheets) To UBound(arrSheets)
    Set ws = Sheets(arrSheets(i))
    hdr = ws.PageSetup.CenterHeader
    hdr = Left(hdr, InStr(hdr, Chr(10)))
    ws.PageSetup.CenterHeader = hdr & "Annual Report " & CHANGES HERE?! ActiveWorkbook.Sheets("Sheet4").Range("=A20").Text
Next i
Application.ScreenUpdating = True
End Sub