Jack London's short story "To Build a Fire" ends in death. For that reason, it's difficult to call any epiphanies a character has in the short story "life-changing." However, there certainly are revelations the protagonist has that temporarily improve his situation and eventually help him to die in dignity and with knowledge.
The greatest epiphany the man has is that, when temperatures are seventy-five degrees below zero, fire is essential for survival. He first has this epiphany when he is trying to eat lunch and fails. He fails because he needs to remove his mittens, but his fingers become frozen too fast and too numb to be able to grip his biscuit. However, in this one moment, he is able to save himself and sustain himself by successfully building a fire.
Sadly, when he gets wet in the creek, he is unable to successfully build the fire he needs to be able to survive. One foolish decision after another prevents him from successfully building a sustaining fire, such as the fact that he built the fire under the limbs of a snow-covered spruce tree, and snow soon fell and smothered the fire. The harder it gets to build a fire, the more he understands the value of the fire. We can refer to his realization of the value of fire as an epiphany. As the narrator phrases it, "He cherished the flame carefully and awkwardly. It meant life, and it must not perish."
His final epiphany is realizing just how foolish he has been. He realizes he should have listened to the old-timer's advice that cold is dangerous and that one should never venture out alone in fifty-below-zero temperatures. His final epiphany is expressed when he says, "You were right, old hoss; you were right." Furthermore, his final epiphany about his foolishness allows him to die peacefully because he realizes that, so long as death was going to happen, "he might as well take it decently."
Hence, all in all, the man realizes that mankind is not as strong as it thinks it is; nature is the dominant force. His realization of the foolishness and weakness of mankind allows him to die peacefully.