Spock slingshots backwards (in a ship other that the Mega-prise) to a point before Nero arrives when the two universes are still one. Then Spock slingshots forward into the future of the prime universe (his universe) to a point immediately after he left the future, Where the universe branches, he moves into the correct branch.
Sounds so easy a child could do it.
If time travel was that easy, everybody
would be doing it.