How to install android 2.2 (froyo) on Nexus One

The easiest way would be to install ROM Manager from the Market.

It is a fantastic application written by the guys that created CyanogenMod. From this app, you can download and install vanilla ROMs and custom ROMs with a few clicks. Highly recommended.

If you un-root your phone you should be able to tell it to re-check for updates and then re-apply the update.

Hopefully you used something like a recovery image to backup your phone before you rooted it, if so then you should be able to restore that backup to get yourself back into an un-rooted state. (depending on the type of backup you took originally, restoring it may or may not overwrite your current user data, make sure you use all the available tools to backup any data you care about before restoring)

Now you're unrooted you can go through the normal Android process to check for OTA updates:

You can see if there's a pending update by clicking the System Updates option at the top of that screen (this will only show the available updates if your phone already knows about them, it doesn't go back to Google/network).

There is a way to force it to go back the Google/the network if an update you're expecting isn't showing:

• Go to the homescreen.

• Tap on Dialer - to open the dialer screen.

• From the dialer type *#*#checkin#*#* (that is *#*#2432546#*#*)

• After a few seconds, you will receive an alert notification, accessible from the homescreen window shade "checkin result success".

• Return to the homescreen.

• Press the Menu key.

• Select Settings.

• Scroll down, and select About phone.

• Select System updates.

• If an update is available, you should see the update downloading.

• When the download is complete, you will be prompted to Install (now or later).

• You can now re-root your phone (after backing up again) to get your Hebrew interface back. If applicable you should be able to re-use the tools you backed your data up with to get it back again.