Universal System, I Can't Upgrade

The protagonist Ding Yuan is reborn into a parallel world with the same name and surname. The world was invaded by a game called "Demon Realm" in the original world, and everyone awakened to the system. Ding Yuan was cursed by the gods after awakening the SSS-level system to kill the gods. He was unable to upgrade and became an F-level waste. But the talent mutates, and the accumulated experience points can be converted into real damage. You can say I'm a coward until I don't have enough experience, but when I have 1 billion experience in hand, even God will praise me for my heart.