How to downgrade IOS 8 from my iphone?

I don't have much space on my iphone with a measly 5.4 gb and the latest update is taking up almost all of it. How can I downgrade the software to an earlier version that won't hog all my space? Thanks.

And yes, I've tried to delete many things to accommodate this update, but it is not enough I'm at the bare bones and I still can't hold that many photos/videos which is why I got the phone in the first place.

You can't. Apple have stopped signing any firmware bellow IOS 8.1.3 meaning it is impossible to downgrade.

Fully back-up your iPhone to iTunes on your computer. Make sure that you save everything of high importance.

Start deleting the Apps / Photos / Messages / Songs etc from the iPhone till it has the maximum amount of space. Turn on Wi-Fi and 3G Data and update to the newest version of iOs.
So here all your Apps / Photos / Messages / Songs etc. Are preserved and you also get to update your iPhone to the latest iOS.

Can't. As Ducky said, the opportunity is gone since all but the latest version of iOS is no longer signed for.