This guide only affects jailbroken iDevices, you can upgrade normally if you are not jailbroken. This method of upgrading will not retain your jailbreak. YOU WILL HAVE TO RE JAILBREAK. NOT ALL PEOPLE WILL GO INTO A BOOTLOOP. THIS GUIDE IS A 100% SAFE WAY TO UPGRADE, YOU WILL NOT BOOT LOOP WITH THESE METHODS. So, iOS4.0 is amazing and you are probably going to want to get it as quick as possible but there is a little trick to successfully upgrading to this new firmware. When you are jailbroken, upgrading to a new firmware through the "Upgrade" option in iTunes will cause your iDevice to enter into a bootloop. This guide will tell you how to upgrade your previously jailbroken device from 3.1.X to 4.0 through iTunes 9.2. Step 0 (OPTIONAL)) Check out this thread to see your compatibility for your iDevice. http://www.ifans.com/forums/showthread.php?t=292579 Step 1) Upgrade your iTunes to version 9.2 (or the newest version). You can upgrade by visiting http://www.apple.com/iTunes/ Step 2) Connect your iPod Touch or iPhone to iTunes and sync your iDevice normally. This will automatically create a backup that you will use when you restore later. If you have already downloaded the 4.0 firmware, visit the bottom of this guide for more instructions. Step 3) Once you have successfully created a backup, click Update on iTunes. Your 4.0 firmware will download - this should take around 15 minutes on a quick internet connection. The download is 366.5mb. Step 4) Once you have downloaded the firmware, unplug your iDevice so you cancel the update. Now, plug your iDevice back into your computer. Step 5) Once you are connected, make sure that your iDevice is on. We are now going to enter DFU mode. Step 6) To enter DFU mode, hold your power button for 10 - 13 seconds. You should hold it past the "Slide to Unlock" prompt. Step 7) Once your iDevice is turned off, hold your home button. Do not release your finger off of the power button. Step 8) Your iDevice should blink white for half of a second. You should then hear a du-du on your computer and your iTunes should recognize your device as a device in Recovery Mode. Step 9) Just hit restore and you should instantly start restoring. Because you have already downloaded the firmware, the restore should be instant. Step 10) Wait around 10 - 15 minutes, you should then be restored. Step 11) When your iDevice reboots, iTunes will ask you to set it up. You can now restore from a backup and your iPod will have your same settings and applications, just on 4.0! Use the following instructions if you have already downloaded the 4.0 firmware. Step 3) Once you have created a backup, Shift-Click the Restore button in iTunes and it will popup a window. Locate the firmware .IPSW file, make sure you have the corresponding firmware for your iDevice. Step 4) iTunes will start to restore your iDevice. It will also restore your iDevice. That is why we created a backup manually. Because 4.0 is the newest firmware, you do not have to redirect your hosts folder and contacting Apple to sign the firmware is fine. Step 5) Wait for the restore to finish and you will successfully be on 4.0 firmware. Step 6) When your iDevice reboots, iTunes will ask you to set it up. You can now restore from a backup and your iPod will have your same settings and applications, just on 4.0! I hope that this guide helped you upgrade successfully to 4.0 without entering a boot loop! If you have any questions, post here and I will help you with the upgrading.