You cannot. Trade with UpGrade to get Porygon2. Trade Porygon2 with Dubious Disk to get PorygonZ, but you have to trade either way.
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Try the GTS trick.
Give porygon an upgrade and trade it it evolve it. And of course you have to trade it back. Try this in Pokémon go!
There is no legitimate way to evolve it without trading.
Well that depends if you just looking for for your pokedex then just look in route 216 for porygon2 but i think there is a possible way to make porygon evolve i recommend training it until it evolves. hope that helped. porygon can only evolve through trade it must also hold up- grade ( PS. any1 who helps this answer i thank u.)
This item can be found in the game it can be used to evolve a Porygon to Porygon2 by trading it to someone.