The easiest thing to do is use the Copy Runtime Geodatabase to File Geodatabase tool in ArcToolbox 10.3. I've found I can only make this work if I have the source files on the C: drive.
The .geodatabse is a SQLite database with Esri's special sauce, so you can rename the files to xxx.sqlite, xxx.sqlite-wal, xxx.sqlite-shm and open them in any SQLite editior if you want to look at the guts.