just pacman -U /var/cache/pacman/pkg/package-X.Y.Z.tar.xz or install the downgrade script for a better experience. not sure about timeshift, it sounds like a backup tool to me.
just
pacman -U /var/cache/pacman/pkg/package-X.Y.Z.tar.xzor install the downgrade script for a better experience. not sure about timeshift, it sounds like a backup tool to me.