How I level up hero 30 to 31?

Your heroes can be upgraded but will need to be 'evolved' at fixed levels in order to progress on and upgrade more.

You need to evolve at level 10, 30, 70, 130, 210, 310

Evolving requires the use of Combat manuals that you earn through playing (and replaying) the campaign puzzles challenges. The higher the level you reach in the campaign the higher the levels of combat manuals that you will receive.

On any hero tap the Evolve button at the bottom of the Upgrade tab to see the requirements.