I just finished updating my Apple iPhone 4 smartphone to the latest iOS 5 and things went smoothly as they did last time, with the process taking less than 30 minutes; however there are a few things to bear in mind to reduce potential issues.
(1) Make sure you have already installed iTunes 10.5 prior to updating your phone and that you have already restarted your machine as prompted by the Apple application. Failure to do so could cause umpteen issues for the update process.
(2) Plug your iPhone via USB to your computer and if you are using a laptop make sure you have enough juice so that the computer doesn't freeze or shut down during the installation. iTunes should detect your Apple device. If not, then shut down everything and restart your computer and repeat the steps.
(3) Once your device has been identified, click on the appropriate icon and the "check for an update" button, which will start the installation process. Note that if you use your device on several computers, you will need to perform the update on your MAIN computer, the one where syncs are carried out.
(4) The rest is only a matter of pressing a few more buttons to confirm the installation and the process should go on automatically. Do not interfere with your iPhone or iTunes during that time or the synchronisation process might be cancelled. Your device will be restarted several times.
(5) Please be patient. Apple says that the whole process could take as long as one hour to be completed. There are reports that things did not go down as well for some users reporting that iOS had bricked their iPhone.