How do I download the iOS?

google it.

Do I have to install it through iTunes to my iPhone?

if you have the itunes you should be able to plug in your iPhone and reset it… Know that this will set it back to the factory default and all data will be lost.. if your iphone is not working correctly AND YOU DON”T WANT TO RESET… sending it to apple may be the only thing you can do

nice work stealing that phone! and computer? Marvin

You can download IOS on your Iphone 3GS. Visit the site http://www.downloadios.net/ to download your IOS. But make sure whether your Iphone get unlocked using permanent unlocking solution. Because if you unlocked your Iphone using temporary unlocking solution it will get locked to its network when you update or upgrade it. If not unlock your Iphone using remote unlocking service from the site http://www.mobileunlocksolutions.com/apple-iphone-3gs/rs1wp9/ ,which is a permanent unlocking solution. Then you can upgrade IOS on your Iphone 3GS it won’t get locked again.