Saves are stored at two different locations. One is the game's own save directory, the other is the platform specific save location:
Python:
# game/code/System/options.rpy
136:## Windows: %APPDATA\RenPy\<config.save_directory>
138:## Macintosh: $HOME/Library/RenPy/<config.save_directory>
140:## Linux: $HOME/.renpy/<config.save_directory>
145:define config.save_directory = "SexNote-1561590144"
Ren'Py stores save files simultaneously to both locations, but other than that, they aren't kept in sync. So either rename the save file so it occupies a currently empty slot (the naming scheme is '<page>-<slot>-LT1.save') or make sure to delete your own save from the platform specific save location as it takes precedence over the game's save directory.
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Simply create a rpy file with a name of your choosing in the game's game direcotory and drop the following into it:
Python:
init 1:
$ config.rollback_enabled = True
Restart and enjoy!