I've never been able to find these files, but for some reason you're on stock but have TWRP installed as a recovery, then you can easily upgrade stock to B14 or B15 without having to go through the over the air updates, since these are modified to remove checks and to flash completely in TWRP.
So if you need to go back to stock B13, here's your easy upgrades.
Edit: For each of these updates, you will lose root. For safety I would reboot to let the upgrades go, reboot to TWRP, and reflash your choice of SU.
So if you need to go back to stock B13, here's your easy upgrades.

Edit: For each of these updates, you will lose root. For safety I would reboot to let the upgrades go, reboot to TWRP, and reflash your choice of SU.