Upgrade to ultimate edition?
The ultimate edition is a special bundle offer, which is the deluxe edition + gems at half price. It’s exclusive through ArenaNet and cannot be upgraded to. You can however upgrade to the deluxe through the gem store.
If your HoT purchase was within 30 days ago however, you could request a refund, then purchase the ultimate.
It’s worth pointing out that the only benefit the Ultimate edition offers is an extra 4000 gems. Which is why you can’t upgrade to it. (The cost to upgrade would be different to the cost of having right out purchased it, therefore losing Arenanet money on the gems).
So while you can’t upgrade to the Ultimate edition, you can upgrade to deluxe, gaining all the items you may have missed out on, and then make a purchase of gems, effectively getting the same thing as Ultimate.