If you don't have proof of purchase, you can contact the retailer where you purchased the camera and ask for a copy of the receipt. If you have purchased it from an e-commerce website like Amazon or Croma, you can download the invoice of your purchase by logging into your account. If you have purchased the camera from the official GoPro web store, you can download the invoice receipt online or provide your email address (the one you used for the purchase) to GoPro customer support and they will automatically fetch your order details.