I got a notification for updating Nuget Package Manager and no matter how many times I tried it kept failing with this error:

all Error : VSIXInstaller.SignatureMismatchException: The signature on the update version of ‘NuGet Package Manager’ does not match the signature on the installed version. Therefore, Extension Manager cannot install the update.

If you are sailing in the same boat..  Here’s what you need to do:
– Start VS in admin mode
– Go to Extension Manager and uninstall NuGet
– Restart VS
– Start VS
– Go to Extension Manager and install new version of Nuget

You can get the latest version of NuGet from here.