https://wiki.archlinux.org/index.php/Arch_Linux_Archive

replacing your /etc/pacman.d/mirrorlist with the following content:

Arch Linux repository mirrorlist

Generated on 2042-01-01

Server=https://archive.archlinux.org/repos/2014/03/30/$repo/os/$arch

Then update the database and force downgrade:

pacman -Syyuu