Unless the Readme - Install notes state otherwise, you can install the firmware you want. So, it seems that you might be able to go directly to 1.13.
Please note: Do not perform the update on the wireless network. Connect a network cable to the router and disconnect from the wireless network. Wireless networking can be turned off, at the wrong time, during the update.
Addendum:
It seems that the SecureSpot feature in firmware 1.12 does not seem to be included in version 1.13. Can anyone confirm this?