Regular Future Stars can’t be upgraded. Their stats are permanent, remaining unchanged until the end of the game.
However, a minimal selection of players receives a Future Stars item which can be upgraded. Each Upgradable Objectives Player will have a dedicated objective that you can work through to earn the four versions of their Future Star item, simulating that player’s progression throughout their career. After unlocking the first version of the player, their subsequent items can be earned by completing various objectives with the latest unlocked version.