Unable to Add Proof of Purchase to Product That is Already Registered

Hello, I recently received a pair of the Blackshark v2 Pro's for Christmas. I registered them for a warranty, but didn't have proof of purchase at the time I registered them. My brother-in-law just emailed me the receipt today, however I am unable to modify the registered product to add proof of purchase. Additionally I don't see an option to delete the product I already registered to start over. Any help would be much appreciated. Thanks!