Updating my iPhone 4 through itunes?

I plugged in my iPhone 4 into my laptop. Itunes popped as a usual and the syncing process occur as usual. I went to "my iphone" and then clicked on "summary". Once there, there was an option "update".It says if I click the update button, it will update my phone to a newer verison of the iphone. (version 5.1.1)

If I click this button and update my iphone, will my data… (pictures, messages, contacts, apps… Etc) be erased?
I've read that if you update your iphone, some data will be erased? Is this true and if so, is there a way to prevent this?

Itunes generally gives you a warning if your data and media are going to be erased during an update. A little popup will tell you of the risks.

Worst comes to worst, Itunes also has the ability to backup your phone, I know it does it for my ipod everytime it syncs. If you update the phone and then import your previous data back it should be fine.

No problem. Update only updates the OS. If you have any concerns, with the phone plugged in, on the left of iTunes, right click the iPhone. Choose Backup. This will store the image of the phone just in case the update is flaky. If you need to restore, it, do the same but choose Restore from Backup.