Before you start, there are some things to understand. One, if you unlock the bootloader on your HTC Sensation 4G, T-Mobile will most likely take the warranty on the device and pour gasoline on it, light a match, and watch it burn to ashes. Also, this is not a process for the Android newbie to undertake. Make sure you know what you are doing and the first thing to know is that you need to download the firmware OTA update that brings version 1.45.531.1 to your Sensation 4G. If you haven't yet installed the update, you can still get it at HTCDev.com.
source: HTCDev.com via AndroidPolice