1. Extract rom.zip from an RUU

A. Run the RUU.
For example, you want to update your Desire HD with
RUU_Ace_Sense30_S_HTC_WWE_3.12.405.1_Radio_12.65.6
0.29_26.14.04.28_M_release_225512_signed.exe. Run this file and wait till extract (you'll see a bar going from left to right) and after that leave it as it is (don't click next or don't close the program)

B. Go to your PC temp folder. For win7 its C:\Users\\AppData\Local\Temp and for winXP its C:\Documents and Settings\\Local Settings\Temp and look for the last created folder. Open that.

C. You'll get a rom.zip file there. Copy it to desktop or other suitable place. Now you can close the RUU program.

2. Rename the rom.zip file

A. You can't directly update the rom.zip file. You've to rename it according to your phone model number. Look for the model number in the serial sticker under battery. For Desire HD its PD98100, for Wildfire S its PG76100 etc. Take first four letter from this model number and then add IMG. So, for example for Desire HD rom.zip should be renamed as PD98IMG.zip and for Wildfire S it should be remaned as PG76IMG.zip

B. Now put this renamed file to a microSD card.

3. Update the ROM

A. Insert the card to microSD slot of the phone.

B. Power on the phone in HBOOT mode
with microSD inserted. For most HTC android phones you have to press
VolDown then Power to go to HBOOT mode. Wait for a while, phone will
read the zip file and will ask you for update confirmaton. Read the
screen instruction and press required button.

C. Update will start. Be patient and wait till being finished.

!!! CONGRATULATIONS !!! YOU'VE UPGRADED YOUR PHONE !!!

Common Errors:
* CID Incorrect : means you are trying wrong region ROM. Use correct region ROM or use Gold Card or make your phone superCID first.
* MODEL Incorrect : you're using other phone's ROM. you can't do it.
* Main Version Low : means phone's HBOOT version is higher than the ROM in your SD card. Use same or higher HBOOT rom's or S-OFF you phone.