a week ago
Welcome to the community and thanks for posting.
When I look on Samsung's website it asks for the Invoice / Order / Account / Receipt Number. When you upgrade you would have been sent a series of emails and this is where you'll find it.
If you've deleted the emails then it may be possible to send them out again. If it's not then a proof of purchase letter can be sent but this will take up to working days to arrive.
Just give the team a quick call on 789 from a Virgin Mobile or 0345 6000 789 from any other line and they'll be happy to help.