1. the first thing you need to upgrade the firmware is..... the actual firmware.... i reccomend this site linksys info
they have plenty of reviews of firmware and support for non-linksys firnware
2. the second thing you need is a computer connected via ethernet to the router in question
3. before flashing some people reccomend backing up configuration and then resetting to default settings
4. finally we get to the action.... open the web interface (192.168.1.1) and go to administration, then firmware upgrade
5. browse to the firmware file and then hit upgrade
6. DO NOT interrupt this process as you risk turning your router into a VERY expensive paperweight.
7. youre done! enjoy!