I have pidora and I want to go back to Raspbian. I have no other computer, so all the steps need to be done on the raspberry pi.
If I can, how?
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If you have a 2nd SD card it can be done. You would download the image from raspberrypi.org to your pidora pi, then use dd to write the 2nd sd card with the image (you would connect it via a usb adapter). Directions can be found here. Put the newly imaged SD card into the pi and you should be all set.
To download image via the command line:
Directions on how to mount the SD card via usb are here.
I'm not recommending this product, but here is what the SD -> USB adapter looks like. You should be able to get one anywhere you can buy an SD card.
If you get a cheap SD card, you can always do the reverse and overwrite the card you have pidora on once you have raspbian installed. Just make sure you get a card big enough to hold raspbian + image file.
Hope that helps.