Tales from the Darkside (1984)

Richard Hagstrom is dismayed with the way life has treated him (a rude son, a shrewish wife). His nephew Jonathan gives him a birthday present: a homemade word processor. Hagstrom soon discovers that anything he types into the processor becomes real, and no one but him knows the difference. He begins with simple experiments, but then wipes out his son from existence. Even his wife is unaware of the changes. As the machine overheats into self-destruction, Hagstrom types in one last story: he makes sure that he and Jonathan's mother married and that Jonathan was his son. As the word processor destroys itself, Hagstrom's final wish becomes reality.