To update NuGet in Visual Studio to the latest version you can use the Extensions Manager.
- From the Tools menu select Extensions and Updates.
- Then in the dialog that opens select the Updates tab.
- Then select Visual Studio Gallery.
- Select NuGet Package Manager for Visual Studio and click the Update button.
If you want to install NuGet 2.8.1 you can download the Visual Studio installer (VSIX) from NuGet’s CodePlex web site