To evolve a character to the next star level they must be at the max level for their current star level.
Star Level 1 = Level 20 max
Star Level 2 = Level 40 max
Star Level 3 = Level 60 max
Star Level 4 = Level 80 max
Star Level 5 = Level 100 max
Step 1 - Select the character you wish to upgrade
Step 2 - On the Stats tab, tap on 'Evolve'
Step 3 - If you have the necessary evolution shards, tap on the gold cost
Your character is now back to level 1, with a new higher max level.
Tip: If you do not have enough evolution shards, you can tap and hold on the Evolution Materials you are missing to discover where they drop in the game!