Trade it whilst its holding an Up-Grade
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Just Evolve Porygon(1). And To Evolve Porygon(1),You Must Trade It.
Porygon doesn't evolve at a level. You need a special item to make it evolve.
In Pokemon Platinum, Porygon-2 can evolve into Porygon-Z using the Up-Grade. I do not believe it serves any other purposes.
To evolve Porygon to Porygon2, you will need to have it hold an Upgrade and trade it. You can get Porygon-Z by having Porygon hold a Dubious Disk and trade it.
Try the GTS trick.