How hard is it to replace an iPhone 4 screen?

My friend said I could have her old iPhone but it needs a new screen. Are the hard to replace yourself? Or should I not try it?

100$ if you buy the screen from apple. You pay them to install it and pay for the screen. It's around 100$.
Under warranty its like 20$

I wouldn't do it yourself if I were you. You might just end up messing the whole thing up, like my cousin! I would bring it to an apple store for them to fix it, like marcus said. First reason is because you are guaranteed a professional, clean and fresh screen replacement. And second reason is because I'm sure they won't mess it up. And if they did, (not 100% sure) I'm pretty sure they would replace it with another iPhone free!

You can follow this step by step tutorial video