What will happen if i upgrade iphone 3gs software from 3.1.3 to 6.1.3?

I have iphone 3gs version 3.1.3, when I'm connecting it to my pc, its asking to upgrade software to 6.1.3. I want to know what will happen If I upgrade this to 6.1.3, my airtel sim card will work or not?
one more thing, I'm not able to donwload softwares on my iphone softwares like whatsapp, markets on mobile news hunt etc…

Please help

Added (1). How to know iphone is unlocked from apple or not?

If your iphone is unlocked from apple the proper way your sim will always work no matter what. And whatsapp will work if you get the latest software.

  • What will happen if I try to upgrade my jailbroken iPhone 4S to an iPhone 5? I recentley jailbroke my iPhone 4S and after a while I decided to upgrade to an iPhone 5. What will happen if I take my phone to Verizon for a new iPhone 5? Will they say no because its jailbroken? Please answer as soon as you can. It would help if you have had experience.
  • What will happen if i change my sim from my iphone 3gs to a atrix 2? I have an iphone 3gs from At&t, i'm going to buy a atrix 2 on ebay. If i swap the swim from my iphone to the atrix 2 will there be any kind of fees? Do i have to change my data plan?
  • Unable to update iphone 3gs software? I just got an iphone 3gs and i'm trying to JB it so i can use it on tmobile but the software is on 3.1.2, when i try to update it to anything new it says "iPhone2.1_4.3_8F190_Restore.ipsw" was not copied to the iPhone (insert name) because it can't be played on this iphone." says this with later updates as well. Any help?
  • Is ''Iphone'' software only or hardware and software? I've never had an Iphone. I always see it anounced in an absolutely black device, such as this one: http://pective.com/m/iphone-5 I wonder whether, when you're going to buy one, you go to the apple store and buy the whole thing, or look for a Nokia or Sony smartphone and then go to the Apple website and buy and download the software into your device.