Did you use the default username and password in the router users guide? If that didnt work, did someone else set the router up that might know the password?
According to the version 4.3 user manual under advanced settings, the username is blank and the password is 'admin' by default. I think thats the default for all versions, but its not a sure thing.
If that doesnt work, press and hold the reset button on the back of the router for 30 seconds while the power is on, and that should reset the router to all of its default settings including the admin user name (blank) and the password (admin). You might have to unplug and replug the power after doing this to make it take effect.
Once you get in, go to the firmware upgrade section and send it your new firmware. The router should reset itself again and within a few minutes be available to you. I'd recommend that you log in again and go back into the administration section and do another 'reset to factory defaults', let it restart, and then go back in and configure any options that you need to make the router work, like login information for your ISP (if any) or any other required setups.
I like to do a factory reset before and after a firmware update on routers. Its not unusual for some crap to be hanging around in the routers nvram before or after an update. The double reset makes sure we get rid of anything hanging around before and after the upgrade.