how to upsert data into mongodb with mongo c# driver 2.5

Problem: There are 3000W+ data in SQL server need migrate to mongo DB, some of those data already in mongo DB, so need to update the data.

Now I use Mongo Driver 2.5 version in C# Console App and can insert multiple documents into mongo DB one time, but I have no idea to update those data already in mongo.

Thanks, advance for your help!

Insert Many with Driver:

public async Task InsertManyIntoMongoAsync(List<T> documents)
        if (documents.Count == 0)
            Console.WriteLine($"No Data!");
        await collection.InsertManyAsync(documents);