Directions are the same for all FW updates and are on the web, however here is the short version.
Use IE or FF to save off the routers current configuration file, if you have special settings or used my sticky to configure the router. This saves time after the update of manually going back in setting it up again.
Update to the version of FW you want.
After the Update, Select the Factory Reset option.
After the Factory Reset, Upload the save configuration file, if you don't then test the internet connections and WiFi.
I would set up the Basic/Internet/Manual Primary and 2ndary DNS for IP addresses from the WAN section in Status/Device info section. Sometimes these don't auto fill in with addresses. Let us know how this goes.