Do not try this if you have an unlocked bootloader or if you dont know what your doing! I'm not resposible if you break or brick your phone!
If you're allready on a rooted .181 or .184 and OTA update is available for you just update your firmware. If there is no OTA update available for your customazation version follow these instuctions:
- flash back to 3.0.A.2.181 generic with the flashtool
- Use Gingerbreak or Superoneclick to root your device
- Update your Arc using a OTA(over the air) update(Go to settings->About phone->Software update)
- Enjoy your rooted 2.3.3!!!
Update 1: it seems that the OTA update is not working if you did any modifications to/deleted system files so it's adviced to always flash the .181 firmware provided in this post
Update 2: if you have any battery problems after flashing the .181 firmware(if it's only showing 1%) removing the battery for a few minutes should solve this
Update 3: thanks to ricstc we now have a video tutorial on how to follow these instructions
Update 4: if you want you can keep your user data by unchecking the whipe user data in the flashtool while selecting the correct ftf file. this will cause a lot of error message's on 2.3.2 but just click them away and continue the procedure, after upgrading again to 2.3.3 the error message's will be gone!
Update 5: this methode still works with the new 4.0.A.2.368 firmware! if you don't get the OTA update with the provided .181 firmware try flashing the .184 firmware and try the OTA again
At the moment there are a lot of battery problems when you downgrade from 2.3.4 if you have any problem follow this or this thread for a possible solution