This option lets you upgrade or change your routers firmware. To upgrade your firmware, download the latest version of tomato for your specific device, or download another 3rd party firmware you would like to upgrade to. Then click Browse, and navigate to where you downloaded the firmware file (should be a .bin or .trx; if its .zip or .rar, extract the archive first) and select it.
Select "After flashing, erase all data in NVRAM memory" if you would like to reset all of your settings after flashing. This is recommended to prevent any configuration issues or conflicts, but it will erase all your settings and thus you will have to reconfigure the router from scratch. When upgrading between TomatoUSB versions, you can check the changelog because it always mentions to clear NVRAM when it is really required. If you are upgrading from different kernel versions (K24 to K26 or viceversa), the clear is required.
Then click upgrade, and do not touch the router! Let it upgrade and wait for it to complete.
This tells you what version of the Tomato firmware you currently have installed, and what build version.
Shows you how much RAM your router is currently not using. RAM us used by services such as FTP server, VPN server, and just general router usage.