I'm copying and pasting SMOKED's instructions from another thread:
1. Connect phone to pc with usb cable.
2. Drag top menu bar down, click 'connect usb' and click mount.
3. Open PC Companion---it might tell you at the top you have an update available. If not go into support zone and click start. This should find the update if its' there and launch SEUS.
4. When told by SEUS, power down phone.
5. Unplug usb cable from pc, leave in phone.
6. After a minute or so press and hold back button on phone.
7. Still holding back button plug usb into pc.
8. Seus will connect to your phone and begin the download.
9. Seus will tell you when to let go of back button.
10. Follow Seus' instructions to complete install.
If PC Companion doesn't find the update. Just launch SEUS and follow steps 5-10 above if SEUS says there's an update. If it says your phone is up to date then the 2.1 isnt' available. You can check the sony blog thread to find your phone's kit number to see if it's been released.