How to change my username in my iPhone settings?

I need to know how to change my Facebook username in the settings on my iPhone. I went to the settings, I went to the Facebook button and it says "There was a problem accessing your account. Please reenter password for ___:" and that's not my Facebook name anymore. I really need to know if I hit the big red "DELETE ACCOUNT" button, would that delete my ENTIRE account or just change something in the iPhone where I could reenter all of my current info?

Your help is greatly appreciated.

See if there's A Log Out Button Hit Log Out and Than it Should Bring you to the Login Page where you Can Sign Into your Account