how can I delete the save data?
This game doesn't use the usual \appdata\LocalLow folder that some other Unity-based games use. Nothing wrong with that, but in order for you to delete or backup your saves, you need to use the windows registry via regedit.
Proceed ONLY if you're comfortable enough to modify your registry.
Open regedit and search for the following directory:
HKEY_CURRENT_USER\Software\PinkTea\Together Again
If you want to delete the config/save data, right-click on the "Together Again" folder and choose delete. It will ask you to confirm deleting the registry, so choose yes.
The next time you launch the game, it will be like the first time you launched it.
If you want to backup your save/config data first before deleting it, right-click on the "Together Again" folder and choose export, then choose a location to store your backup registry file.
Next time you want to import your save file, just double-click on the .reg file and confirm importing your data into the registry.
That's it.