Ideally, before doing anything else:
- Make sure the device is completely powered off
- Use a plectrum or another, preferably plastic, tool to pry open the back cover along the edges:
- Touch a metal surface to discharge any static electricity
- Remove SD card from slot:
- Connect SD card to computer through a USB or built-in card reader
- There are many partitions on the card, and most of them are in the format unrecognized by Windows, so you will get prompted many times to format them. Answer "no" each time.
- Make an image of the SD card. You can use VConsole's Flash Drive Image Creator (graphical interface), or Chrysocome's (command-line) DD for Windows, or another tool of your choice. Ideally, do it one more time, and make sure the images match, for example with fc /b (command-line tool included with Windows).
- Optionally, clone the image to a new SD card and keep the original one as a backup. You can use Flash Drive Image Writer, or DD again, from the pages linked above.
- If you want to use a larger SD card, clone the original image first, and then resize the user data partition (the only one with the FAT32 filesystem) to fit all available space with a tool such as GParted. If you foresee installing more apps, you can perhaps increase the size of the /data partition (the size is 500MB by default, note: I didn't test this).
- Put the SD card (original or cloned one, depending on your choice) back in its place and power on the device to make sure it works. The back cover seems to be a tight fit: you need to press all 4 corners at the same time to make it snap on. In my case, heating it up a bit with a hairdryer also helped.
If you don't backup the SD card you should still be fine, as long as you:
- Don't accidentally overwrite the bootloader (if not changing the boot logo, the risk is generally low)
- Don't mess up the partition layout
- Don't suffer from a hardware failure of the included SD card (most likely of the 3)
So this step is optional.