Recently I thought of updating the Flex SDK, for my flex builder 3. Thought of sharing the steps with all as I took quite some time to discover how to do it.
First download the flex sdk from here. Then unzip the files and name the rename the folder to 3.4.0 or simply keep as it is. Copy this folder and paste in the following location, C:\Program Files\Adobe\Flex Builder 3\sdks\. After that run flex builder. Go to Project –> Properties, then click Flex Compiler tab. Then click on Configure Flex Compiler… link. Then click Add button, browse for the Flex SDK location (eg: C:\Program Files\Adobe\Flex Builder 3\sdks\3.4.0), automatically Flex SDK name will come, or you can rename to some custom names also. That’s all we have successfully installed the new flex builder SDK. Now select the new SDK check box and click Apply. That’s all. All our applications will be compiled using the new Flex SDK.
Happy Flexing… 
