For those who haven't got the update yet.
This is how to do it on a Desire:
Steps to install Froyo on HTC Desire
Step 1: Download the Android 2.2 firmware for the Desire from http://liveimage.htc.com/OTA_Bravo_Froyo_HTC_WWE_2.09.405.8-1.21.405.2_release4dua3bcrut4wrv30.zip
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.