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You can use the following code to modify a particular element within a nested array using the Mongo C# driver.

var collection = db.GetCollection<YourModel>("YourCollection");

var filter = Builders<YourModel>.Filter.Eq("_id", new ObjectId("ObjectIdHere"));
var update = Builders<YourModel>.Update.Set("YourArray.$[i].PropertyName", "NewValueHere");

// Replace 'i' with the index of the element you want to modify in the array

var options = new UpdateOptions { ArrayFilters = new List<ArrayFilterDefinition> { new BsonDocumentArrayFilterDefinition<BsonDocument>(new BsonDocument("i", index)) } };

var result = await collection.UpdateOneAsync(filter, update, options);

In this code, YourModel is the model class for your collection and YourCollection is the name of your collection in the database.

The Filter variable selects the document that contains the nested array you want to modify. The Update variable specifies the modification you want to make to the element of the array.

The ArrayFilters variable provides a way to filter the nested array elements. In this example, we are filtering by the index of the element we want to modify, which we pass in as a parameter.

Finally, you call UpdateOneAsync on your collection with the filter, update, and options variables, which will modify the selected element of your array.