ArsTechnica published a guide about manually updating the Nexus One to Android 2.2

Google unveiled Android 2.2, codenamed Froyo, at the Google I/O developer conference last week. The new version brings significant performance enhancements and a number of compelling new features. Google intends to roll out the new version to Nexus One users in over-the-air (OTA) updates over the next few weeks. Users who want to get their hands on the latest version right away, however, can manually update their handsets.

In a concise message published in the XDA Developer forum, well-known Android modder Cyanogen posted a link to Google's official Froyo update package a compressed zip archive that can be manually installed on unmodified Nexus One devices.

  How-to: manually update your Nexus One to Android 2.2