When you first receive a Card, it starts at Level 1. You can increase its level through Upgrading.
Upgrading a card boosts its Attack and Defense stats, with the extent of the increase depending on the card's rarity. Cards can be upgraded up to Level 5. To upgrade a card, you'll need Dust, which you can obtain through Recycling. Some combos can only be researched with higher-level cards.
To Upgrade:
Menu: Select "Upgrade Cards" from the Main Menu. Select Card: Browse your available cards to find one to upgrade. Use the Search/Sort/Filter options if you’re looking for a specific card. Upgrade: Once you choose a card, you’ll see the Dust cost and the stat increase. You can preview upgrades up to Level 5 and see its Fuse Ability once Fused. You can choose to upgrade one level at a time or pay the full Dust cost to upgrade directly to Level 5. Pay Dust: Tap "Upgrade" at the bottom of the screen to spend your Dust and complete the upgrade. If further upgrades are available, you will have the option to continue. NOTE:
Cards currently in your Deck will be labeled. If a card is already at Level 5, you will have the option to Fuse it with another Level 5 copy of the same card to unlock its fusion power.