lavendar town.
The name changer guy is in eterna city next to the pokemart
In Jubilife City
Lavender Town.
Because you got Blue Version- an updated version of Green. (I have no clue Why they changed the name)
the name rater is in goldenrod city to the left of the third gym (Whitney)
In lavender town
Slateport in front of the contest place
The Name Rater is in Eterna City, on the right of the PokeMart.
In eterna city he is a old man in an apartment building he claims he is a name rater select the Pokemon to rate the name then he asks you if you want to change it
On every Pokemon game, there is a name changer. He will allow you to change your Pokemon's name, either with a small fee, or free of charge.
Lavender Town
i think you can generate codes for that using pokesav.