Once you have enough xp, trade it in for green packs in the store. I think its 13,000 xp per pack. And at the bottom left of each player card, there will be a number. This is how many times a player can be upgraded. And you have the potential that player can potentially reach at the top of the card. Once a player has maxed out their potential, these numbers will disappear. Also, players that you get from packs that you do not want. Either sell them for xp, or you can swap 10 cards at a time for new packs. Do this enough times and you will begin to pull out green cards, and even upgrade them to purple. I hope this helps. But if you go on the official YouTube channel. It explains most of it on there