No one really changes her she has Damon's and Stefan's blood. When she drowned under the bridge instead of dieing, since she had that vampire blood, she turned into a vampire. That's how Damon and Stefan turned into vampires. With that girl's blood, sorry forgot her name, and when they both stabbed each other in the heart they turned into vampires instead of dieing. So the vampire blood from both Damon and Stefan and then being killed turned Elena into a vampire.