I don't know how to do what you asked about for a text file,
but this psuedo code will give you an idea of how to accomplish
what you describe.  Substitute the actual workbook names into
the array and enter your changes for the header into the
quotes where PROJECT NAME GOES HERE appears and it should
change the header for all the sheets in all the workbooks listed in the array.

Sub HdrFtr()
  Dim WB As Array
  WB = Array(WB1, WB2, WB3, WB4,...WBn)    '...WBn means the number of
workbooks.
  For i = 0 To UBound(WB)
     For j = 1 To WB(i).Sheets.Count
    With WB(i).Worksheets(j).PageSetUp.
                    .CenterHeader = ""        
                    .CenterHeader = &"Century Gothic" &12 &B "PROJECT _
                     NAME GOES HERE"
     Next j
  Next i
End Sub

This can be modified so that a user can make the changes by making an entry
into a pop up input box rather than having to change the code.

Sub HdrFtr()
  Dim WB As Array
  WB = Array(WB1, WB2, WB3, WB4,...WBn)    '...WBn means the number of
workbooks.
  newStuff = InputBox("Enter revision to project data", "CHANGE DATA")
  For i = 0 To UBound(WB)
     For j = 1 To WB(i).Sheets.Count
    With WB(i).Worksheets(j).PageSetUp.
                    .CenterHeader = ""        
                    .CenterHeader = &"Century Gothic" &12 &B & newStuff
     Next j
  Next i
End Sub