Every few months I need to bulk update all of my VS2015 projects to use the latest version of a certain package. Because I spent another 10 minutes stuffing around to get the correct syntax, I'm putting it in here so I won't forget again. The following command recursively loops down looking for *.sln files and runs a nuget update against them.

for /R %f in (*.sln) do nuget update "%f" -id packagename

If the solution's projects do not contain the package id you will get this for each project:

Updating 'project name'...
WARNING: Unable to find 'packagename'. Make sure they are specified in packages.config.

If the solution's projects do not use Nuget then you get:

Scanning for projects...
No projects found with packages.config.

If a solution's project is updated you get this without a following warning message:

Updating 'project name'...

The resulting output from processing many solutions and projects can be quite cluttered and difficult to read to see what really happened.

UPDATE NOTE -- December 2015

Solutions containing web projects don't seem to upgrade correctly using the technique described above. I found I had to open these solutions in Visual Studio and manually update the packages. I haven't diagnosed the details of this issue yet, so this is just a friendly warning.