To export your project click menu button on the project row on the list of projects view and tap "Share project file". Depending on installed applications, you should be able to save the file to Google Drive, Dropbox, send it via email and so on.
To import previously exported file, click it in a file explorer app. That should start Floor Plan Creator and import the project. Clicking the same attachment more than once will import multiple copies of the project.
If Floor Plan Creator is not started after you click .fpc file then open the application, click menu button and select "Import". Depending on the OS version you will either see a standard file picker or Floor Plan Creator will search for all .fpc files on your device and show them on a list. Click project that you want to import.