After each battle, you will accrue vehicle specific experience. If you have an 'elite' armored vehicle, you will have the option to convert the vehicle specific experience to free experience (experience that can be used by any vehicle in your garage) by using gold, at a rate of 1 gold per 25 XP converted.
Please note that Elite vehicles have a golden laurel wreath around their vehicle type in the upper left area of their vehicle icon.
Log onto the game client:
Click on the "Convert Experience" button by the top right hand corner of the client: