his trophy is story related and will unlock after getting Edna to recount the events of her time travel.
As soon as you arrive at Edna's house in the Alternate 1931, you will need to go up to her front door to activate the tripwire. She will yell at you from the window, then leave. Activate the tripwire again and choose any dialog options. Activate the wire a third time, but now tell her you spent the day together. When given the option, tell her you have brought her something. When she asks what, interact with Doc. Tell her that he is Emmett Brown and she will come down stairs. Now you have the task ahead of you to re-jog her memories about what happened to Hill Valley when she went back in time. After she fully recounts the events, you will leave this time frame and your trophy will unlock.
For more information, refer to the trophy walkthrough under "The Future isn't Written ".