You can improve your players through methods such as leveling up, upgrading to expand their level cap, promoting a player to a higher grade, or reassigning bonus stats.

Level-Up
You may level up a player by collecting a certain number of duplicates of the player's cards depending on the player's rarity and then consuming them.
Each time a player levels up, their stats are increased.
Other enhancement methods may require leveling up the player to a certain level.

Upgrade
A player must be upgraded when they reach a certain level.
If the Upgrade is successful, the player becomes stronger.
A successful Upgrade will increase the player's stats.
Upgrading players will also increase the number of skills a player can equip.

Promotion
Promotion can be done by consuming Star Pieces that match the player's position.
A successful Promotion will increase the player's stats.
Promoted players will also be able to equip higher-level skills.

Reassign Stats
Reassign Stats allows the redistribution of bonus stats acquired by leveling up, upgrading, or promoting a player.
Bonus stats are distributed based on probability depending on the growth type when training a player.
By reassigning stats, all of the bonus stats that have been distributed so far can be redistributed.

Both Upgrades and Promotions have success rates.
Clovers can be used to increase the success rate. (Upgrade, Promotion, Reassign Stats)
Also, note that only the currencies that you have spent will be consumed if you fail.