Players can be upgraded when you collect enough copies of their card. You can receive copies of player cards from packs, as well some through the Mid-League Training Camp.
Training can be a great way to improve specific players. Upgrade your Stadium to receive more Training slots to upgrade more players at once.
Make sure to only play fit players or their performance and ratings will be negatively impacted.
Player Fitness is regained by resting them for the specified time shown on individual player screen via squad screen.