In the application after you downloaded the flight numbers from PSA airport website (Settings menu) you can add the flight number for starting a new day.
First start a new day, then in the Preorders menu manage the preorder (You'll see what you have left). For change tap the € icon and add the change. For exchange the change you need to give check the actual exchange rate and update it in the Settings menu. Further information, suggestions or any problem with the application contact me at ervinhun[at]gmail[dot]com