Can I upgrade to an IPhone 5s?

I have an iPhone 4S and my birthday is this month. I don't know how phone upgrades work. If I pay the 200$ dollars for the 5s can I switch it with my current iPhone

If your contract is over, yes you can.

But if you're in a contract you'd have to pay an extra fee to upgrade.

September 20th the iPhone 5S and 5C go on sale.

iPhone 5S with a 2 year contract:
16 GB - $199
32 GB - $299
64 GB - $399
Colors: silver, gold, and space gray.

iPhone 5C with a 2 year contract:
16 GB - $99
32 GB - $199
Clolors: blue, white, pink, yellow, and green.