iPhone 4S Software update question

each time I plug in my 4S, a window opens saying "A new iPhone software version (6.1) is available for the iPhone.  Would you like to download it and update your iPhone now?" 

I have canceled each time as I am afraid that it will erase all of my contacts, phone numbers, emails, etc.  Can you confirm that if I do that, I won't lose my info that is currently on my iPhone?

Thanks!