You can't change the nickname someone else gave a Pokemon(meaning through the Name Rater), however, if the Frogadier's name is it's normal name in Japanese( it'll look like this; フロガディエ), then just evolve it and its name will change into the name of the Pokemon of the language you set the game to(in your case, most likely English).
If the Frogadier has an actual nickname(anything not フロガディエ), then it will stay as that name no matter what.
But like Staka~ said, you can just breed it if you want.
Hope this helped! ;)