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).
You can back up all of these files just as you would back up any other computer file, using an external drive, a cloud drive, or some other method.