Alanna's style has evolved over time. She was a real nerd in high school but eventually became a seductress.
At the beginning of this clip, she wears the outfit she wore in high school and she destroys it piece by piece, symbolically saying goodbye to her old style.
After that, Alanna wears her typical new outfit. Since these clothes are a bit worn out and she plans to buy a new one, she also tears this outfit.