If you have US-RC30/UK-RC8 or higher, you will first need to downgrade your phone to a previous version. (Skip these steps otherwise)
Format your phone's SD card to FAT32 mode:
Hook your phone up to your computer using a USB cable and then wait for the notification to show up in your title bar of your phone.
Click the notification, and then click "Mount".
A new removable disk should show up on your computer. Right click it and select Format, and select FAT32 as the file system type.
Download and unzip the RC29 or RC7 image file. Copy the DREAMIMG.nbh file to the SD card. (RC29 for US, RC7 is for UK)
Turn the device power off.
Hold Camera button, and press Power button to entry bootloader mode. You should see a gray/white screen with instructions to flash your phone with the update on your SD card. If you don't see that, make sure you followed the instructions properly.
As per the on-screen instructions, press the Power button to start upgrade procedure. DO NOT DO ANYTHING TO INTERRUPT THIS PROCESS.
After it is finished, perform the restart your phone.