Enhancing a superstar becomes available when a bronze or silver superstar reaches both their maximum level, and maximum move level.

Should you have the necessary coins and tokens, pressing on the “Enhance” button will raise the maximum obtainable level of your superstar, and the maximum obtainable level of their moves. This will be shown by their color class increasing by one level. 

Bronze enhances to Silver
Silver enhances to Gold

Enhancing will be replaced by Evolving if you reach the maximum level on a gold class character. Check out our “Evolving” article for more details.