How to download iOS 6.1.3 on my iPhone 4?

How can I download it? Everytime I try to it says "this update requires at least 2.7 GB of avaliable storage". What does that mean?

You have to delete some content because you have less than 2.7GB capacity left on your iPhone.

You will have to delete some content because there's not enough space on your phone to download new software