They get in the ghost's wife's house, and the wife starts to say "back up here" when they just get in. Is it "right here in the back of the room" or "let's try to summarize somehing"
wife-Just for a minute
Ok, ok,back up here .You're telling me you're talking with julian?
Like his dead ghost or something?
mel- Yes -Even I feel his presence
In this case, "back up" means Melinda went too fast or that the listener made an assumption that proved to be untrue, so she needs to go back to an earlier part in their conversation so she can understand better.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.