How to upgrade my iPhone 3GS with ios4.2 to latest ios?

I was using my home pc to connect my iPhone 3GS and sync. All my contacts, apps and other necessaries are in that pc. But now I left my home to the hostel. I have another laptop now. How can I save my things and sync it to the new laptop? Also I need upgrade my ios 4.2 to the latest version. How can I do it via iTunes. How much size will it be? Please help me in a step-by-step way. Also what will I do if I lost the internet connection during up-gradation?

Apple has been restricting certain iOS features to certain devices since iOS 4, and iOS 6 is probably the most confusing example of this yet, mostly because there are now many different iOS products with varying performance levels.

As the oldest supported device, the 3GS misses out on the largest number of features: it can't use the 3D Flyover or turn-by-turn navigation features of the new Maps app (restricted to the iPhone 4S and newer), Panorama mode (iPhone 4S or newer), or the offline reading list feature (iPhone 4 or newer). Features from older iOS versions that didn't make it to the 3GS are still not present in iOS 6: these include location-based Reminders (iPhone 4 and newer), Wi-Fi Personal Hotspot (iPhone 4 and newer), FaceTime (iPhone 4 and newer), AirPlay Mirroring (4S and newer) and Siri (iPhone 4S and newer).

These missing features generally don't change the operation of the phone-they're simply not present. One app that does change is Maps: on newer devices, getting directions from "current location" invokes the turn-by-turn navigation feature. There's also a button in the lower-left corner of the screen that you can tap to bring up all the steps in your route.

On the 3GS, getting directions from your current location brings up a list of driving directions that you can swipe through to see where you're going-swiping from the left brings up the next step, and swiping from the right brings up the previous one. There's also no button to tap to bring up all of the steps at once, which seems like an odd omission to me. (If you have a newer phone and would like to see how this works, you get the same interface if you use anything other than "current location" as your starting point when getting directions.)