Rearrange pdf pages (swap odd pages and even pages)

I have a pdf and I am in need to replace even and odd pages. If I want to be more specific, I must replace (2n-1) page numbers with (2n) ones (1-->2, 2-->1, 3-->4, 4-->3, and the like). How should I do that? Is there any free software that can do this job for me?