How to update my iPhone 4S to iOS 7 without losing everything?

I have to update my phone using my computer because I don't have enough gigabytes to install the upgrade on my phone. I'm only doing the update because I was told by Verizon that I have to update my phone or else I can't connect to Wi-Fi which I haven't had on my phone in like three weeks, so I don't want to lose anything. How do I safely back up my iPhone 4S and then how do I go about with updating it after that?

Back up everything to iTunes trough your usb cable, install the latest update, get all your files back, be happy.

Use Dropbox to back up photos or temporarily delete large apps. Install the update and re-download apps and photos.