Depends on your platform.
Here's the desktop platforms.
Windows: %APPDATA%/RenPy/[some game folder]
Mac: ~/Library/RenPy/[some game folder]
Linux: ~/.renpy/[some game folder]
And for Android, you need to go into storage\android\data\ and locate the com.naughtyroad.* folders, and then go into files\saves, but you can only do so from a computer with the device attached, those folders are shielded on Android itself.