To do this, Bell had to sing in three voices, one for each of the stages of Anna's life: from little girl to angsty teenager. This allowed Bell to sing in both the upper and lower range of her soprano voice.
In "Frozen", Anna is the younger sister of princess Arendelle, Elsa. Elsa possesses the magical power to create snow and ice. After accidentally hurting Anna with her powers, she mostly hid in her room, while Anna's memories were wiped of the incident. This caused a strain in the girls' relationship.
In the film, Bell did not sing Anna's parts. Her performance surprised the crowd and showed her versatility as an entertainer. Watch the video before Disney takes it down.