Please help me to update my divice

You can't. iOS 6.1.3 is the end of the line for your phone.

The compatible devices have been posted on Apples website since June.

Iphone 3gs is not compatible with ios 7.

because, it was a old device from 2009 almost 6 years old, the 3gs had a darn long run, but, it just wasn't compatible. plus, you can always go get a new phone, if you just wanna hang on to your 3gs, go ahead, i aint stopping ya so, but iOS 6 is still very compatible and will run almost everything is 7 can run