Too many of us on here are constantly having to write the same answers to the same questions. If you are reading this because you have upgraded to ICS and are having problems with some applications and battery life, this thread should help you not having to ask the same questions again....
I am assuming you are using Kies to upgrade as anyone using Odin will probably have the experience to sort the issues themselves anyway....
1 - Before upgrading BACK UP your phone. To do this you need to download Kies from here http://www.samsung.com/uk/support/us...tware/KIES/JSP. Install the software then open Kies. Connect your phone to PC using the supplied data cable and WAIT until your PC loads the drivers for your phone. After a few moments you will see a message that says "your device is ready to use". At this point you should see your Kies change to show details of your phone. IF you have an upgrade available you will see the message saying so on screen. Otherwise you will see a message saying "your phone has the latest firmware", this means no update available, yet. At this point press cancel on the upgrade message because you need to BACKUP first. Go to the BACKUP tab in Kies and select the components you wish to back up. Wait until this process is complete then disconnect your phone from Kies and close Kies down.
See also Tufftoad's excellent backup thread here http://samsunggalaxysforums.com/show...up-your-phone-)
2 - FACTORY RESET. Turn off your phone and wait for the vibration to notify you it is completely shut down. Then hold the following buttons down together in this order. Vol +, Home key, Power ON. Hold all buttons until you see the Samsung slash screen then let go, wait until you see a menu appear on screen. Use Vol - key to select "Clear cache" and apply using power on key, wait then use Vol - key to select "factory reset", accept the warning using power on key then execute using power on key again. Wait whilst your phone reboots. You can skip through the set up screens at this point until you get to your home screen.
3 - UPGRADE TO ICS. Unlock your phone to the homescreen then connect cable to phone and PC again. Kies will start and offer you the upgrade. Accept the upgrade and follow the onscreen instructions. DO NOT TOUCH YOUR PHONE AT ALL UNTIL UPGRADE IS COMPLETE. Once Kies has finished your phone will reboot to ICS version 4.0.3.
4 - Disconnect phone from PC and REPEAT step 2 to clear cache and factory reset again.
5 - Connect phone to Kies for the last time and go to "Restore" tab in Kies where you can restore the backup of anything you created in Step 1. Check your accounts and add anything as required. Most likely will be the need to add your Google and Samsung account details as these don't appear to get restored by Kies (you may be lucky though).
6 - Job done, enjoy ICS.
7 - If you want the official ICS wallpaper pack, see here http://samsunggalaxysforums.com/show...ICS-Wallpapers
8 - If you experience poor battery life, see here http://samsunggalaxysforums.com/show...-Apps-and-Tips
You may also need to recalibrate your battery by allowing the battery to drain fully until it turns off. Then, recharge in one go with phone still off. Do not use the phone at alll during this calibration charge period.
9 - SGS2 ICS manual and user guide here http://samsunggalaxysforums.com/show...Manual-for-ICS
Hopefully this will help with most of the common issues, please feel free to post anymore tips and guides and remember the forum has a SEARCH function. Use it and you will probably find the answer to your question much more quickly than posting the same question again and waiting for a reply