This is the most common method for home users and small businesses who have purchased a retail license but need to apply it offline. ESET provides a web-based tool to bridge the gap.
Extract the .lf file from the zip (do not rename it). Copy this .lf file to the root of your clean USB drive. Safely eject the USB.
You will use a separate, online computer to generate the license file. This PC does not need ESET installed, but it does need a modern web browser.
You must repeat this process periodically (ESET recommends at least once a week for critical systems).