Depending on whether your phone is locked and won't accept foreign updates, you may not have to root your phone, but you will have to unlock it by using the goldcard method.
Luckily this website helps you do it in a few simple steps:
Note: these steps will NOT root your phone.
Download: klutsh's Goldcard Tool
- Format your SD card to FAT32 by going Settings > SD Card & Phone Storage > Unmount SD Card then Format SD Card
- Launch the goldcard tool as admin by Right click on App > Run as Admin
- Connect your phone to your PC and make sure USB debugging is enabled (Settings > Applications > Development > USB Debugging)
- Click get CID (Try clicking MMC0 if MMC1 fails)
- Click the link to open the goldcard generator, copy the CID from the tool into it. You will get an email.
- Enable USB Mass Storage on your phone.
- Click 'Refresh' in the app and select your micro SD card.
- Click 'Load goldcard.img' and select the file you received by email.
- Click 'Patch MMC'
- You now have a goldcard!
- Now you can downgrade using the Generic RUU.
After these steps you've changed your Android version to a generic one and you can use the generic update to get it to 2.1.
Kudos to klutsh from MoDaCo and the person who made the site!