Simply go to the home screen, click on "File Manager" and you should see an "Upload File" button at the top of the screen. The button will allow you upload GCODE or X3G files depending on which driver you selected in your printer profile.
The print file will be uploaded directly on your AstroBox.
If you upload an STL, you will be redirected to the astroprint.com cloud slicer to slice the model. Afterwards, the STL and GCODE will be stored in your cloud account and the GCODE will also be available to download to your AstroBox.