[header] How to evolve porygon [title] Obtain an up-grade. [step] You will need to give this item to porygon to hold. When porygon is traded while holding an up-grade, it will evolve into porygon2. 
You can get an up-grade in gen 2 (gold, silver, crystal) and later. [title] Equip porygon with the up-grade item.