New Game Plus in Nier: Automata is covered on this page.
New Game Plus
When beat the game for the first time, you will earn Ending A. Save your data to begin a second playthrough with all of your gear and progression intact. Load up this save to start the subsequent playthrough.
Once you've defeated the game 3 times, save the game. This new save file will let you access Chapter Select and Debug. From Chapter Select you can launch any part of the story you have unlocked. The menu will now show you your progression. Side Quest progression will carry over from playthrough to playthrough so you don't need to redo them. Debug Room lets you fight enemies in a room to practice combat.