It seems when an item in Pokémon Legends: Arceus is only effective on one Pokémon the rarer it is to come upon. This rule particularly goes for the Upgrade item, as most are likely seeking it to only evolve a Porygon into a Porygon2. In total, there are two main places where the item can be obtained, with one able to give you the Upgrade in just a moment’s notice.
The best method to obtain the Upgrade is through the Trading Post at Jubilife Village. The post is located just outside of the Training Ground, and will offer the Upgrade for just 1,000 Merit Points. To get these points, you’ll be required to head to anywhere in the wild and find satchels laying around. As each satchel will give users a little less than 100 Merit Points, make sure that you collect at least 11 of these bags before you head back to the post.
Later into the game, players can discover Upgrades when defeated Pokémon drop items in the Space-Time Distortion. Although the rate of the item dropping isn’t great, it is certainly much higher than it is searching for it in the wild. Once obtained, you’ll be allowed to evolve your Porygon, no matter its current level.