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?
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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.