Save files are in:

Please note that some of these folders are hidden folders. you can access them either by typing into the navigation bar or show hidden folders.

Mac:
Users/[username]/library/cache/Yangyang Mobile/The Letter/State
Or
Users/[username]/library/Application Support/Yangyang Mobile/The Letter/State

Windows:
C:\Users\[user_name]\AppData\LocalLow\Yangyang Mobile\The Letter\State

Linux:
/home/[username]/.config/unity3d/Yangyang Mobile/The Letter/State

Android:
{Storage}/Android/data/com.yym.theletter/files/State



Some of the stuffs are stored in the registry data.
Registry Data:

Windows:
Open regedit (Registry editor) in run or search

navigate to:

HKEY_CURRENT_USER -> SOFTWARE -> Yangyang Mobile -> The Letter

click on that folder to see the registry data.