iOS - Replace Siri with Google Assistant
Siri and Google Assistant are similar offerings from Apple and Google and are deeply integrated in their respective mobile OS. It is not possible to replace one with the other.

While Siri is exclusive to iOS (along with other OS by Apple macOS, watchOS and tvOS), Google provides the Google Assistance app for iOS.

Google Assistant app for iOS can be downloaded on iTunes App Store.

On an additional note, as Siri is deeply integrated into iOS, there are various features exclusive to it, which Google Assistant cannot perform on iOS.