Using the SOjbect Id of an existing Lead to set a new field value.
Id leadId = new Id("00Q2800000fgxkKEAQ");
Lead lead = new Lead(leadId);
lead.LastName = "Smith";
GenericMongoService service = new GenericMongoService(Lead.SFType);
SaveResult saveResult = service.UpdateEntity(lead);
if (saveResult.success)
{
Assert.AreEqual(leadId, (Id)saveResult.id);
}