Warning: Pls read their guide carefully before upgrading, and there is NO warranty for any damage to your devices ^^.

Start here:

http://www.dd-wrt.com/wiki/index.php/Index:FAQ

Download firmware:

http://www.dd-wrt.com/site/support/router-database

If you do not know which version you should download, read this:

http://www.dd-wrt.com/wiki/index.php/What_is_DD-WRT%3F#File_Versions

I persionally use this version:

http://www.dd-wrt.com/routerdb/de/download/Buffalo/WHR-G54S/-/dd-wrt.v24_vpn_generic.bin/1966

I have upgrade successfully my WHR-54GS to the latest dd-wrt fr and as in their guide, you should remember to hard reset (restore to default settings) your WHR-54GS before upgrading.

(….)

Then connect to http://192.168.1.1/Upgrade.asp

remember to choose “After flashing, reset to: Reset to Default Settings” before click on “Upgrade”

Its process should take about 300 seconds to complete….

That’s it!

Good luck!