How to Get IOS 7 On My IPhone 3GS?

I know its an old phone and i'm getting a new one soon!

Go into settings and look for system update

Don't put ios7 on your device. It is not fast enough to handle it. You will like it at first, then you won't be able to do basic things on your phone. I would not recommend upgrading. But if you do, check for an update in settings or jailbreak your phone.

Iphone 3gs does not support iOS 7 because its an older phone. You need at least an iPhone 4 for iOS 7. You could get something that look like iOS 7 by jailbreaking it but i wouldn't do it. IPhone 3gs is pretty slow now so if you jailbreak it it will be super slow. IPhone 3gs support iOS 6 and its still usable so just leave it on iOS 6.

You can't 3GS is not compatible as its to outdated.