How Do I Add Proof of Purchase To My Already Registered Product?

Hello there, I have a question on how to add my proof of purchase to my already registered product, so that I may be able to get the 2-year warranty I am eligible for. I registered my product without my proof of purchase because I couldn't manage to find it through my emails, and thought I could just edit it and add the proof of purchase later. I managed to find my proof of purchase a day later, and now looking, I can't find an edit button or add button to the proof of purchase for my already registered product. I am hoping there is still a way for me to add my proof of purchase, so if anyone knows anything I would really appreciate the help! Thanks!