This assumes that any change to sell A1 in your worksheet will update the header/footer option. The code should be added to the worksheet that contains the A1 cell that will be updated.
I've included all 6 locations that a header or footer can be entered, so you would have it for reference, but I would suggest you clear out any line that doesn't actually need updated. Move the
MyUser variable to which ever section you want it to show in (the quotes would not be required, as shown in the example).
Code:
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
Dim MyUser
MyUser = Range("A1").Value
If Target.Column = 1 And Target.Row = 1 Then
With ActiveSheet.PageSetup
.LeftHeader = MyUser
.CenterHeader = ""
.RightHeader = ""
.LeftFooter = ""
.CenterFooter = ""
.RightFooter = ""
End With
Else
End If
End Sub