in Song view, tap for the menu, tap the key the song is in (bottom right). At the top left of the Transposition window you will see the global transpose key (assuming it is in Concert key, you will see a 'C').
Tap it, the choices will show. For your alto sax, tap the Eb. Tap the chart to exit. You will now see the chart transposed down to your Eb key, but the player will remain playing in concert key.
There are tutorials in Settings on a variety of topics.
Have fun!