You will need a spare microSD card. These are the steps to restore the orginal content of the emmc card:
1. dd the "emmc_installer.img" to the microSD card
2. insert both the emmc and the microSD cards into the board and set the switch to boot from the microSD.
3. Boot the board. The emmc card will be written automatically with the original contents taken from the microSD. Wait until the process is finished (when the green led goes off and the board will shutdown. cooler goes off too).
Now you have the original Android system and u-boot on the emmc card. You can boot from the emmc or continue with the steps to replace the Android with Arch Linux ARM.
Cheers!