I'm beginning to play with updating my MongoDB collections using C# based Windows Services and tooled up this small application during testing in order to figure out the best way to do the updates I need to. This iteration is not LINQ based, rather it uses the native Mongo driver capabilities to update a single key/value pair for a single entry in a collection using command-line arguments.
You will need to download and install the MongoDB C# driver, which can be found here: https://github.com/mongodb/mongo-csharp-driver/downloads
Usage: 'MongoUpdater.exe server database collection searchKey searchValue updateKey updateValue'