The best way to overcome it [the fear of death]—so at least it seems to me—is to make your interests gradually wider and more impersonal, until bit by bit the walls of the ego recede, and your life becomes increasingly merged in the universal life. Since you asked --
Let's take it apart. He's talking about overcoming the fear of a personal death -- the death of your own ego, being, memory, identity. If one is afraid of death, one is afraid of ceasing to be. One asks, "How can the world exist without me?" It's kind of unimaginable. So, it seems Russell is talking about losing your personal identity and having the Zen hotdog: One with Everything. How can you fear death if you experience yourself as part of everything?
He says as much in the last sentence. It sounds possible in theory. On the other hand, one could "Rage, rage, against the dying of the light."