Saving an audio file with a modified time step using AVAudioEngine in Swift

Question

I'm currently trying to change the time step of an existing audio file from the Document folder, and then override the old file with the modified one using AVAudioEngine. I have the first part working using AVAudioPlayerNode and AVAudioUnitTimePitch. However, I have no idea about the second part. Can someone point me in the right direction?

Thanks.