You can change his squad numbers...

1.Click at the player and the pencil button on top
2.Feel free to edit your player's details, including his name, nation, and his squad number as well. This cost 1 token but this is good to add some realisticness into the game such as adding real-life, famous player names like Lampard, Rooney, etc.
3.Click Save.

That's it, you're done!