You need to go to PC->User->YourName->AppData->Roaming->RenPy
Save the game before choosing what you want (Serve/Kill/Protect/To be left alone...) or before the last choice of having sex with Nameless. Those are the only two unchangeble choices.
After choosing one option, if you want to explore the other option you need to delete the files inside RenPy folder and Save folders and paste in RenPy folder the old folders.
Be aware that you could loose all your progress if something goes wrong.