How to download iOS 8 without an issue?

I tried to download iOS8 about three times and they all failed. It would download for about five hours eachtime. I'm updating it on my phone not on my laptop.

If you're having trouble downloading iOS 8 on your phone, then I highly recommend that you download it onto your computer first, then you install it on your phone. Just back up your iPhone, then update.

I also think it's better to update through computer. It needs more space and power in iPhone.

You can check this video for explanation