The upgrade Firmware menu allows you to upgrade (or downgrade) the firmware on your FreshTomato hardware to another Tomato firmware build.
You can find detailed information and step-by-step instructions on the flashing process in the HOWTOs section on the InstallingFreshTomato page.
Clicking the Browse… button, will allow you to select a suitable firmware file to flash (write).
You are very strongly urged to enable the After flashing, erase all data in NVRAM memory checkbox. Variables often persist in NVRAM after a firmware flash. Persistent variables can cause system instability or strange behaviour after the upgrade/downgrade. They may also cause wrong menu text or images to be displayed, or cause text or images not to be displayed.
After you select a firmware build file, that build's filename will display to the right of the Browse… button. Clicking Upgrade will start the actual firmware flashing process. For detailed information on choosing a build to flash, see the section titled How Do I Choose a Build? (build naming scheme) on the FAQ page.