In Neural Mix Pro, you can easily change the key of any song loaded into the app.
To do this, use the left or right arrow buttons next to the Pitch setting found just below the track waveforms, as shown below. Then, click the button in the center of those arrows for the changes to take effect on the track.
When you click the center button, you will see the key of the track, located above the overview waveform at the top right of the screen, change to reflect the pitch setting you've chosen. To quickly reset the key back to the original, click the center button again to deactivate the change.