It's possible you're thinking of:
* Upgrade: This item is found in Pokémon Diamond, Pearl, and Platinum and is used to evolve a Porygon into a Porygon2. It's not available in HeartGold.
* Up-Grade: This item is found in Pokémon Gold, Silver, Crystal, and their remakes and is used to evolve a Eevee into an Espeon. This is the item you likely meant to ask about!
To get an Up-Grade in Pokémon HeartGold, you can:
1. Find it in a hidden location: It can be found in the Goldenrod City Department Store on the fourth floor, in the back room, next to the Poké Ball vending machine.
2. Trade for it: Other players might be willing to trade for it.
Remember, you need the Up-Grade to evolve an Eevee into an Espeon. To do this, level up Eevee during the daytime while it's holding the Up-Grade.