1) Download the AstroBox image for your Raspberry Pi. It can be found here: DropBox.
Unzip the .zip file, you should see the AstroBox image file, it has a file extension of .img.
2) Download and install Etcher for your operating system from here: https://etcher.io.
3) Power down the printer and disconnect the power cable. Now, remove the Rear back panel of the printer then locate and pull out the MicroSD card and plug the card into your computer .
See the picture for MicroSD card location on printer:
4) Run the Etcher software and Select the .img file that you just downloaded from the Astroprint website. Now, select the SD Card you want to use and click FLASH! Wait for the install to finish.
5) Once the flash is finished, place the MicroSD card back into the Raspberry Pi. Reinstall the back cover, Plug the power cable back in and turn on the printer.
6) All done! The printer should now be on the updated firmware.