You can't install a previous version of a package in our current infrastructure. Even though Emacs’ built-in package manager now supports multiple versions of a package in recent releases, no popular package archive (i.e. MELPA Stable and Marmalade) actually keeps a backlog of previous releases.
Emacs didn't support this for a long time, and now that it does, the demand for this feature seems to be rather low—as it is generally for versioned releases, considering that MELPA Stable is far less popular than MELPA itself. Notably, none has yet volunteered to work on a backlog of releases for MELPA Stable, and consequently there's little incentive for the maintainers to implement it.
You'll have to download, build and install the package yourself. QUELPA with a custom recipe for Magit will probably help here.