To update your phone using Nokia's PC software updater. You need to download the software and have your phone and data cable ready. Here are the steps to be followed-
Step 1: Backup your phone. Generally a firmware update leaves your memory card items untouched, but its better to be safe and backup all your SMS and contacts.
Step 2: Connect your phone via USB and select the PC Suite mode. your phone will be identified and the software will search for an update for your phone. If it finds one, it will display an update button. Click it and proceed.
Step 3: The software will download the firmware and install it onto your phone. If you have a slow internet connection, it may take some time. You can do some other work while this process is being completed. It will indicate when it is finished. Restore the data back to your phone.
This method works for all Symbian phones. Its the most effective method. Do not worry about anything going wrong with your phone due to a disconnection. I've had my Internet disconnect a couple of times while updating and it is safe. The update is installed only after it is verified.