Maybe you need to specify the source of the Nuget - it has to be set to EPiServer Nuget. You could also add the target project (if you have more than one).
Install-Package EPiServer.CMS –projectName MyProject -Version 8.1.0 -Source http://nuget.episerver.com/feed/packages.svc/
It is worth noting that downgrades are not supported. Database schema changes etc are irreversible.
You could also try changing the version in packages.config and run Update-Package –reinstall. But like Magnus said, you will need an old backup of the database as well.
I change the packages.confic and run Update-Package –reinstall. but still i get one Error "NuGet Package restore failed for project Menu: Unable to find version '8.8.0' of package 'EPiServer.CMS'..How to resolve it ?.
I have installed new version (18.104.22.168) episerver CMS in my system. But I want previous version (22.214.171.124). I google it and got some answers,
NuGet Package Manager CMD: Install-Package EPiServer.CMS -Version 8.1.0
but its not work.I want to install 126.96.36.199 Version.