Go to your garage, click the turret for which you want to buy an alteration.
Once you click it, these boxes will show up (green represents the alterations that are currently available for purchase.)
Click once again on the green boxes and you will open their description (e.g. Firebird's high pressure pump)
If you like one of the alterations, click the buy button. Please note that if you want to buy an M1 Firebird alteration you must have M1 Firebird, if you want M2 alteration of Freeze you must have M2 alteration, etc.
Customer support service by UserEcho