Get NSS if you don't already have it:
http://www.b-phreaks.co.uk/index.php?main_page=page_2
Backup any personal files you don't want to lose. I don't remember if anything is deleted or not, but anytime you're dealing with firmware you want to backup your stuff! If you have a microSD card to back up to, do that, or just use Nokia PC Suite to backup to your hard drive. Remove the microSD card before you start NSS. Also make sure you have a full battery or have your phone on the charger.
Basically what you want to do is connect your phone to your computer, let it install all the drivers, then when Windows gets done doing it's thing you can start NSS. Click the magnifying glass at the top right of the screen to scan your phone.
Click the "Phone Info" button, then under Actions click scan. Note that on the bottom status bar your phone has been fully detected.
Now click the Tools button and find the "Factory Settings" tab near the bottom. Click "Full Factory" then click the Reset button. Your phone's screen may turn white for a bit, but that's normal. When NSS says the reset is complete and your phone comes back up, you may disconnect your phone from the cable.
You want to power off your phone at this point, then turn it back on. When the phone comes back up it will be partially debranded! You can reinsert your microSD card and restore your files if needed.
Once you get your cable and start this process, if you have a question don't hesitate to ask me. Keep in mind that this will void your phone's warranty, so proceed at your own risk! It's well worth it though just being able to have MP3s as ringtones and use java apps that need web access.

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