In Safari or in any app that your on? In Safari it's the back arrow on the bottom. But if you wanted to go back to the last app/page you were on, double press the Home screen and the multi-task will pop up showing the list of apps you opened recently, the one on the left being your last app (last screen you were on). Just tap on that app and it'll take you back.
Where is the "back button" for editing contacts/ My old "dumb" phone let you edit a field (e.g., last name) without deleting the entire entry. 4S solutiuon?