How did Yuno become alive again?
In Redial, the Yuno is the Yuno of the third timeline, she inherited the memories of the Yuno from the first timeline. This is explained by Deus that despite the alteration of the third timeline by God Yuki, the persistence of Yuno towards Yuki transcended the timelines.


Combined with the calling of Murmur and that odd feeling she had (love for Yuki), she arrived and freed Murmur, obtaining first timeline's Yuno's memories from Murmur 1.