Update all properties of object in MongoDb

var update = new BsonDocument("$set", new BsonDocument(entityType.GetProperties().Where(p => p.Name != "Id").Select(p => new KeyValuePair<string, object>(p.Name, entityType.GetProperty(p.Name).GetValue(task, null)))));
var options = new UpdateOptions();
collection.UpdateOne<MyTask>(item => item.Name == "cheque", update, options);

this code uses reflection to include all properties of the given object
to the update statement, no need to manually add all properties, as u see the Id is explicitly excluded from the update statement to avoid exception.

You can do that with ReplaceOneAsync instead of UpdateOneAsync.

You need a filter to match the existing document (a filter with the document id is the simplest) and the new object.

Hamster hamster = ...
var replaceOneResult = await collection.ReplaceOneAsync(
    doc => doc.Id == hamster.Id, 
    hamster);