解决办法很简单：
1、在控制面板，卸载程序里，卸载Nuget；
2、如果你使用的也是windows 7系统，要用管理员权限运行VS（否则NuGet卸载按钮不可用），在工具—扩展管理器中我们仍然可以看到NuGet，再次卸载NuGet；
3、直接在扩展管理器中安装最新版本的NuGet即可。
官方解决办法原文如下：
Upgrading to latest NuGet from an older version causes a signature verification error.
If you are running VS 2010 SP1, you might run into the following error message when attempting to upgrade NuGet if you have an older version installed.
When viewing the logs, you might see a mention of a SignatureMismatchException.
The workaround is to simply uninstall NuGet and then install it from the VS Extension Gallery.
See http://support.microsoft.com/kb/2581019 for more information.
