They have a chance to change the past, but also need to make sure not to change so much that the nephew isn't born.
If you're in need of an R-rated sci-fi comedy movie that doesn't take itself too seriously, Hot Tub Time Machine is perfect for you. To sum it up, this movie is the story about a man, his two friends, and his nephew who take a trip out to a ski resort to reminisce about the past. But when they decide to get really drunk in a malfunctioning time machine, they suddenly find themselves waking up back in 1986 in their old bodies. They have a chance to change the past, but also need to make sure not to change so much that the nephew isn't born. It follows many of the same rules you'd expext from a classic time travel movie, but with raunchy comedy sprinkled in throughout.