MongoDB C# Driver 2.0 - Update document

I think you're looking for ReplaceOneAsync():

MyType myObject; // passed in 
var filter = Builders<MyType>.Filter.Eq(s => s.Id, id);
var result = await collection.ReplaceOneAsync(filter, myObject)

To add to mnemosyn's answer, while a simple ReplaceOneAsync does update a document it isn't equivalent to Save as Save would also insert the document if it didn't find one to update.

To achieve the same behavior with ReplaceOneAsync you need to use the options parameter:

MyType myObject; 
var result = await collection.ReplaceOneAsync(
    item => item.Id == id, 
    myObject, 
    new UpdateOptions {IsUpsert = true});

you can use LINQ as following:

await context.collection.ReplaceOneAsync(b=> b.Id == item.Id,item);