Format, clean install, & restore iPhone 3GS I have been using iPhone 3GS for almost 3 years now, and the phone has got pretty slow. I understand that there are many reasons for the phone getting slow, but I want to try to improve its performance, even if it is by a meagre amount, by formatting it completely and installing the latest iOS it can support, i.e. 6.x.x. 

However, I don't want to lose important data like messages, contacts, notes, internet/network settings, etc. by doing a format and clean install. I have the latest iTunes installed, and would prefer iTunes for this process. I know how to update iOS using iTunes and restore from backup, but I am not sure about format + clean install AND then restore all the important data.  

Can anyone please advice me how should I do this task?
Connect the device to iTunes and click Back Up Device.
Once the backup is complete, click Restore iPhone and follow the instructions to restore iOS on your iPhone.
Once this is complete, iTunes will prompt you to restore your backup which will restore all your data back to your device. All your data will be restored to your device except account passwords unless your backup is encrypted.