Nintesndo dsi question?

please answrere. i need help. it may sound wierd but my young sister bit the top screen. now there are colors and lines every where on thetop screen. i dont know what to do. i have a warranty but how do i use it?i bought it online. and .my grandmother will be very mad once she finds out becaue she bought it. i cant fix the screen myself. can i get anew one with the waranty? how do i do that?i bought it online so do i just bring into the store???