If you are using a modded 3DS, you'll need homebrew tools to "inject" these files into your game:
The internet is full of dead links from 2012. Do not download a .exe file claiming to be a save file. You need a .sav file. sonic generations 3ds 100 save file upd
Press to select it, then select New (or a similar backup option) to export your current save data. If you are using a modded 3DS, you'll