From Terminal Cancer Patient to Goddess of Time and Space

While waiting to die after giving up treatment for his terminal cancer, he unexpectedly gets an app that lets him travel through time and space. He can change the fate of his counterparts in parallel worlds in the past, and receive rewards from them in return. When he learns that Blue Star will meet its end in 2046, he must become the Goddess of Time and Space and shoulder the responsibility of saving Blue Star civilization.