Place a card in the center where it says "Select Hero", then either click "Add" to immediately add low-level cards or manually add cards. Click "Fuse" when done to upgrade the card. The card's skill can be upgraded by fusing it with cards that have the same skill. Coins are needed for any upgrading. There is a maximum level when upgrading for all cards.
Fusing a 5 star Merlin card with a card gives the card 50,000 EXP.
Fusing a 4 star Merlin card with a card gives the card 10,000 EXP.