1. Get the official OTA update from http://android.clients.google.com/packages/passion/signed-passion-ota-42745.dc39ca1f.zip, rename it to update.zip and put it into the root folder of the SD card in your N1. The MD5 checksum should be 94f103b38c726fe17a5ff8e7676a737b.
2. Unfortunately, the standard recovery image on a European N1 will not be able to verify this update, so you need to flash the stock US image first. This requires an unlocked bootloader. If you haven’t unlocked your N1 yet, you need to do it now. This will void your warranty and wipe all data on your phone! You can find various guides on how to this on the internet, e.g. here. Be sure to wipe manually after unlocking. All data gets wiped anyway but it’s possible that Froyo won’t boot if you don’t do a manual wipe, too.
3. Now get the stock recovery image from here and install it (as described on that page).
4. Reboot while holding the volume down button and select Recovery from the boot menu.
5. When the screen with the android and the exclamation mark appears, press volume up and power for a moment to get into the recovery menu. You may have to try this combination several times before you get the menu.
6. Select Apply SDcard:update.zip and wait for the update to be installed. The phone will reboot automatically (which can take a few minutes on the first reboot).

Do this at your own risk! Back up all data and settings first!