NieR:Automata has to be one of my favorite games of late, with an amazing story and kick ass game play.
But at the end of Ending:E the game will ask if you want to give up your save to help other players finish the game. I wanted to help but also really didn't want to lose by game save. but it seems that if you just backup the save file before going into Ending:E it can be restored and all of your game data will still be there. You will also now have the ending:E background. This will only work on the PC copy of the game.
You should find you game save in you documents folder under
Save a copy of the "SlotData_0.dat" files, you should have one for every save.
Now just play through the ending, get that sweet background.
Exit the game and copy back the "SlotData0.dat" files into the NieRAutomata folder and now when you start the game you should have your old save back.
This is just a video of me playing through Ending:E, at the end I was able to restore my save to go right back into the game to get the other endings