If your device is already rooted, then simply erase the apk using RootExplorer (or any other similar app), then reboot.
Then simply install the app (older version) directly launching the apk.
This could lead to problems if you are :
-replacing the default launcher (then be sure you have at least a temporary one installed)
-the app you are trying to replace is only a system one (then copy the older apk with RootExplorer, set permissions rw/r/r and reboot once more time)
Edit : true it all apps you’d better to downgrade. Refer to tutorial