to dress up – means to wear a costume or fancy dress (special clothes for an event such as carnival). This phrasal verb dress up is followed by AS or IN.
Examples: What are you going to wear for Carnival? I’m going to dress up as Spiderman. I’m going to dress up in a Spiderman costume.
Another verb with the same meaning is ‘to go as’.
Examples: Are you going to the fancy dress party? What are you going to go as?