"Entanglements" by David Gerrold
The Magazine of Fantasy & Science Fiction, May-June 2015
Description, by Sam Tomaino (SFRevu): "A semi-autobiographical tale in which David, our narrator, receives a gift on his 70th birthday from "Pesky Dan Goodman": "a parellelicon... a quantum resonator... a quawkie talkie". It allows David access to alternate versions of his world, alternate versions of himself, alternate stories he wrote in those versions."