This article with help you determine which Firmware you have on your router and which steps you need to follow on our Support page, it will also help you determine which firmware you need to download when upgrading.
The Vertical Firmware is the first version that we release on our N routers and it looks like the image below:
It´s called the Vertical Firmware because the options are displayed on a side bar, usually given to ARN01154U1 routers.
The Horizontal Firmware is the last version that we release on our N routers and it looks like the image below:
Its called the horizontal firmware due to the options being displayed on a Horizontal Bar, usually given by ARN01154U2 routers.
NOTE: To UPGRADE your router, follow this link for more details about firmware upgrade: How to upgrade my router.