Change of username and latest iPhone Update

I recently successfully changed my Apple Username, following my move to a new ISP. However after having successfully downloaded the latest iOS software onto my iPhone, my phone asked me to complete a few more steps to complete the update. One of which was to enter my Password for the displayed Username. The displayed Username (which could not be altered) was my old Username (the password to which I have now forgotten and unable to receive emails at its' address, ie: from Apple with forgotten password hints etc).

After three failed attempts, I pressed the 'Skip This Step' option and receved the Apple message that my iCloud and Facetime wouldn't be set up correctly as I hadn't entered the required information!

Can someone please help me with what I've done wrong or how I can change the pre-displayed Username information on my iPhone?

Thank you