How do I restart the 2.2 update for HTC Desire?
Settings | About phone | System updates

It should let you know that there's an update available.

If you still have the update.zip file on your SD card, these instructions should do it: http://geekword.net/manually-install-froyo-htc-desire 


  Step 1: Download the Android 2.2
  firmware for the Desire from here.
  
  Step 2: Rename the file update.zip and
  copy it to your microSD card via USB.
  [Note: make sure the file is named
  update.zip and not update.zip.zip.]
  
  Step 3: Power down your Desire. 
  
  Step 4: Hold down the “Volume Down” button
  as you power the phone back on. 
  
  Step 5: A screen should appear showing your
  phone’s system searching for various
  files. Scroll down to “recovery” and
  press the “Power” button. 
  
  Step 6: When you see the triangle with an
  exclamation point symbol, press the
  “Power” and “Volume Up” buttons at the
  same time. 
  
  Step 7: From the menu that appears, select “Apply
  sdcard:update.zip.” 
  
  Step 8: When the screen displays “Install from sdcard
  complete” select “reboot system now”
  and wait for the phone to power back
  up.