You have to trade with it holding the dubious disk. This can be done when porygon2 is at any level.
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give it a dubious disk and trade it
train it
Used on porygon to evolve to porygon2
The only known Pokemon to evolve using the up-grade is porygon. It will evolve it into porygon2.
Just Evolve Porygon(1). And To Evolve Porygon(1),You Must Trade It.