You can't. The name rater won't change names for Pokemon that someone else named... only for Pokemon that you named yourself. This is also true for Pokemon that you traded from another game... any other game, even if it was you playing it, the game can't tell, and so the names are permanent.
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You can rename a pokemon by going to the Name rater
There is a name rater in Castelia city.
you go two the name rater in the town where the Pokemon tower is
you go to the name rater's house. but if it is a trade Pokemon then you cant rename it.
Oh, dude, renaming Pokemon in FireRed is like a walk in the park. You just gotta go to the Name Rater in Lavender Town, like, he's the guy who's all about changing names. Just talk to him, select the Pokemon whose name you wanna change, and boom, you can give your Pikachu a new identity. It's like giving your pet a cool new nickname, but way more digital.