Note: These instructions worked on my own Nexus One, which was purchased contract-free from Google. It has, however, been reported to work fairly reliably on Nexus One models of most stripes, though this seems like an English language update, so proceed with caution if you're not sure this update is what you want.
- Grab the .zip package containing the Nexus One 2.2 update here (direct ZIP link). Rename the file to update.zip, then copy to your Nexus One's SD card.
- Follow SimonNWalker's instructions at the AndroidSpin forums. To summate, you'll have to turn off your Nexus One, reboot it while holding down the trackball button, and follow the instructions to activate "recovery mode", then load the update.zip file up for flashing.
- Give your Nexus One time to reboot, "catch up" to all the new changes, and when things have settled down, you can either head to the Market and search for Adobe Flash Player, or head directly to adobe.com/go/getfp10android to launch the appropriate Market page.
Things look fairly smooth on my Nexus One with 2.2 installed. I couldn't say whether I'm seeing the kind of real, tangible speed boost Google boasted of yet, but Flash does run, and seems to run well enough to get you past the inconveniences of web sites that need it for just one or two functions.