Android 2.2 Froyo has officially arrived, and its first appearance on public handsets is on Google’s self-branded mobile phone, the Google Nexus One. Unfortunately, the current manual update is limited only to those Google Nexus One handsets with build version ERE27 (your Nexus One’s build version can be checked under Settings>About Phone).
The update is also limited to Nexus One phones containing the SIM of select mobile operators. If you have a Google Nexus One that qualifies for the above criteria, or just a handset that supports Android 2.2, it is recommended to wait for the official Over The Air (OTA) update. If you do want to try the slightly risky and modification-filled manual method, then find the steps of how to do so, here. Android 2.2 will be officially launched via OTA update for the Nexus One within the next few weeks.