Hi,
The simplest way is to synchronise your Android device and desktop OpenSong using Google Drive and a free third party app such as DriveSync. This will allow updates on your Android device to be available on your desktop and changes on your desktop to be pushed to your Android device. This is what I do and you can find out more info at:
Alternatively, if you create a backup of your songs, it will generate a *.osb file. This is actually a zip file with a different extension. You can copy the files/folders from this into your desktop's OpenSong/Songs/ folder. Make sure you do not include the persistent database or highlighter files when your create your backup). If you do this manual method, you would need to copy any xml files created in the desktop app OpenSong/Songs/ folder and paste them into your Android OpenSong/Songs/ folder and then allow OpenSongApp to rebuild the song index when it starts (full index), alternatively from OpenSongApp go to Settings>Import>File and select the new XML file created.
I'd highly recommend the sync option at the top as this also synchronises sets.
Gareth