How do I update the OS in my device?
How can I update my Desire S from Android 2.3.3 to Android 2.3.4? Is it possible to update the OS for this model?
madlymad's right, there is no official update. XDA has several 2.3.4 custom ROMs, though. Should you choose to go that route, read a few of the linked ROM threads and then follow the instructions for the one you want.