What will happen if I update to iOS 7?

I have an iPhone 4 which I have been told can update to iOS 7. But if I do, what will happen?

Will any of my apps be deleted or start over or something? Because I have this free music app-NOT illegal-that has all my music on it and I don't want it all deleted.

What about my pictures? Do I have to back them up first? What about my apps WITH pictures?

Notes? Calendar? Will it completely start over?

Will my apps be arranged the same or not? I NEED All THE INFO I CAN GET!

(Optional) can you please tell me how to update? I heard something about connecting it to the computer and updating the AppStore first?

It will not delete anything! It may be smart to backup your phone with your itunes account though. But it does not change anything about the apps, just the layout and how you get around your phone. But I enjoy the update so i would definitely do it!

Wait till the official release on the 20th. You probably don't have to back it up, but you could. To be safer, back it up and update on the same day. It probably won't delete everything

I don't think you can update it on a iPhone unfortunately

Back up everything before updating to iOS 7.
It's easier when you update through iTunes.

Not recommended for iPhone 4

It will be the end of the world as we know it.