Tropy files can be backed up just like any other files on your computer. They live in three places: 1. The project file (.tpy), saved wherever you told the file to save when you created your project, likely in My Documents or a similar folder. 2. The 'user data' folder, which contains any custom templates you might have created:
~/Library/Application Support/Tropy (macOS)
~/.config/tropy (linux) or
%APPDATA%\Tropy (Windows), 3. and the photos (which Tropy currently does not modify at all).