This menu lets you upgrade/downgrade the firmware on your FreshTomato router to another (Fresh)Tomato build.
You can find details and step-by-step flashing instructions on the InstallingFreshTomato wiki 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 flash. 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.
Always check the MD5SUM of any firmware build download to ensure its integrity before flashing.