[Xperia Arc Q&A, Help & Troubleshooting] duridian:
I think the user above means firmware, not framework.

Anyway it's easy, just use Flashtool and official FTF file. As you said you were running CM9 you must already have an unlocked bootloader. These steps are for unlocked bootloader phone and this is how I did it on my Xarc.

Go to this thread:

http://xdaforums.com/showthread.php?t=1330314

and download official LT15i 4.1.B.0.431 Generic Global World (big bold letters, you can't miss it in post #1)

Unrar this file to get the official Sony 4.0.4 FTF.

Then go to:

http://androxyde.github.com/Flashtool/

Click the tab that says "Installation".

(I assume you are running MS Windows)

Download Full Install, Windows Edition - v0.8.0.0

Download Updates, Windows Edition - v0.8.5.0

On your computer run installer v0.8.0.0 first, then run v0.8.5.0 to make it the latest version of Flashtool. Flashtool by default installs to c:\flashtool and doesn't create a shortcut in start menu, so you will have to find it with file explorer (My Computer).

Power off phone. Do not connect data cable yet!

Run Flashtool.

Drag the FTF file onto the Flashtool window.

In the window that appears, leave everything as is, but tick "No final verification" in the bottom right corner.

Click "OK" and wait a few seconds.

When the message appears, hold down the back button on your phone and insert the data cable.

Notification LED should turn green and Flashtool begins flashing the firmware.

Wait until flashing is complete. Flashtool will say "Ending flash session" and say something about phone rebooting when it is finished. Wait about a minute after these messages appear and you can then unplug the data cable.

Now for Clockworkmod:

Go to:

http://xdaforums.com/showthread.php?t=1680793

and download LT15i Arc v2 from first post.

This is the stock kernel with CWM added.

With phone powered off, hold down menu button and insert data cable. Notification LED should turn blue.

Run Flashtool. Flashtool should say "Device connected in fastboot mode"

Click the button in Flashtool which looks like a lightning bolt (the button furthest to the left.) Tick "Fastboot mode" and click OK.

On right side "Select Kernel to flash", find the kernel with CWM you just downloaded and "Open" it.

Kernel flashes in just a few seconds.

You can now unplug data cable and start up the phone. You can give a quick run through the start up settings, language selection etc. before we root. (The only real reason I said to start up the phone is to make it easier to transfer the Superuser install zip to your sdcard from Windows.)

Now root:

Go to:

http://androidsu.com/superuser/

and download Superuser-3.0.7-efghi-signed.zip

from the Eclair/Froyo/Gingerbread/Honeycomb/IceCreamSandwich (2.1 – 4.0.3) section.

In the settings menu of your phone: Menu > Settings > Xperia > Connectivity > USB coonection mode: Choose Mass storage mode (MSC)

Connect the data cable.

When your sdcard appears as a drive in Windows, copy the Superuser-3.0.7-efghi-signed.zip file onto it.

Unplug data cable and power off your phone.

It's important to watch the notification LED for the next bit!

With phone off press the power button as normal when turning on.

Once the Sony logo appears, watch the LED.

When LED turns blue start pressing the Menu button. You have less than 3 seconds to do this!

This takes you into the Clockworkmod recovery.

Use volume up/down move up and down. To confirm use Home button.

Select "Install zip from sdcard" > "Choose zip from sdcard" >

Superuser-3.0.7-efghi-signed.zip > Yes - Install Superuser-3.0.7-efghi-signed.zip

Once the install completes you can go back and "reboot system now"

Your phone should now be rooted.

Good luck.