This video shows you how to flash your Google Nexus One (T-Mobile only) from Android 2.1 (ERE27) to Android 2.2 (FRF50).
Link to download the file for updating from ERE27 to FRF50
http://www.mediafire.com/?k0z44zkjv5z
UPDATE: During the making of this video, an update for froyo was released. The link to update from FRF50 to FRF72 is here also. The steps are exactly the same except you download the FRF50 to FRF72 file instead.
Link to download the file for updating from FRF50 to FRF72
http://www.mediafire.com/?tnjgnmqygnw
UPDATE2: Here's the link to download the latest froyo version FRF83. Just follow the steps in this video! Enjoy!
http://www.mediafire.com/?zymqgjdqini
UPDATE3: you can't update from the stock phone (ERE27) to the FRF83 update. You must first update to FRF50
