How to Extend Norton 360 deluxe if product key is purchased from a store

I purchased Norton 360 deluxe from Staples.

I want to extend my subscription and not lose the two months I have left.

No way to do it online that I could find.

Norton online help is pretty much useless.

I called the 800 number. She wanted to sell me a new key for a lot of money.

Finally gave her the new product key. Then she hung up on me.