Level 7 character upgrade, just like previous upgrades, increases character powers and the number of skill card slots however unlike previous upgrades, it doesn't require any character cards; only requires a certain amount of Gold and some luck.
You can upgrade your characters in the Characters section of the Main Menu. After pressing the upgrade button, you can see the upgrade chance and upgrade attempt price. Character upgrade chance reduces as the character rarity increases. Common Characters have the highest upgrade chance while Legendary and Heroic Characters have the lowest upgrade chance.
There are 5 categories of upgrade chances:
- very low
- low
- medium
- high
- very high
You need certain amounts of the chance to increase your category for each character rarity:
- For Common Characters:
55% chance to reach medium
205% chance to reach high
330% chance to reach very high
- For Rare Characters:
210% chance to reach medium
510% chance to reach high
760% chance to reach very high
- For Legendary Characters:
900% chance to reach low
3000% chance to reach medium
6000% chance to reach high
8500% chance to reach very high
- For Heroic Characters:
900% chance to reach low
3000% chance to reach medium
6000% chance to reach high
8500% chance to reach very high
You can simply attempt to upgrade your character by pressing the upgrade button and hoping that it succeeds or press the plus icon next to the upgrade chance and add up to 10 chance cards to increase your upgrade chance. 
You can choose which cards you want to add by pressing "+" icons on the bottom or simply pressing the "Auto Add" button on the bottom left to add your best chance cards. 
You can obtain chance cards from Player of the Match, Daily Missions, Scratch Cards, and Bundles.
Keep in mind that, unlike skill cards, chance cards don't require any currency to add/remove and aren't consumed when added to these slots. You can add chance cards to see the increase in upgrade chance and simply leave the upgrade screen by pressing the "x" button at the top right, all added chance cards will return to your inventory. However, all selected chance cards are consumed upon attempting to upgrade a character whether it succeeds or not.
Don't worry if the upgrade fails, you can keep trying until it succeeds with the same price.