How to download a previous iOS software?

So I have an iPhone 4, and I downloaded iOS 7 and it took up so much space, so I tried deleting apps, and still didn't have enough. I had to end up getting rid of my music, which I realized was not a good trade off! How do I download a previous iOS software? Like iOS 6.1.3? Any information would be helpful. I'm kind of technology impaired, so if you could break it down step by step that would be awesome. Thanks!

Added (1). If I can't download a previous version, is there a way to create more space on my phone?

You can't. Apple stopped signing 6.1.4 and 6.1.3 on Sept 21st. Every time you try to change the firmware your iPhone (or iPod touch) will contact Apple and either allow your to install it or not. After Sept 21st they stopped letting 6.1.4 and 6.1.3 being installed.

If you want to know what Apple still signs you can go to the website below, which tells you that and has the firmware to download.