Sub retourne_col()
Dim i%, x%, a As Variant, b As Variant
x = 0
For i = 1 To Range("A65536").End(xlUp).Row
If x >= Range("A65536").End(xlUp).Row / 2 Then Exit Sub
x = x + 1
a = Cells(i, 1).Value
b = Cells(Range("A65536").End(xlUp).Row - i + 1, 1).Value
Cells(i, 1).Value = b
Cells(Range("A65536").End(xlUp).Row - i + 1, 1).Value = a
Next i
End Sub