Douluo's Reversal of Destiny

What should you do when you discover that you have the power to change the fate of others? Both people traveled through time due to someone else's mistake. Since we are destined to reach the top, we must turn the tide. (The author wants to give a happy ending to the beautiful memories of his youth. He will not exaggerate, nor deliberately whitewash. He will only try his best to provide a logical condition. How to do it depends on the protagonist.)