There are two ways you can retire units:

  1. Open the Yonder Yacht, right by the Arena. There, you'll be able to select as many units as you want to retire in exchange for Hero Coins.
  2. Select a unit in your Kingdom's main screen and tap on Retire. Confirm to receive Hero Coins in exchange for your retired unit.

Think carefully before retiring a unit! It is an irreversible process. Units of higher ranks will reward you with more Hero Coins. Use these Hero Coins to buy items at the Retirement Shop in the Yonder Yacht!