How can I return to the very beginning of my tax return to change something after I've gone on to other sections?

From within your Federal return, select the Wages & Income tab at the top left of the screen, you should be shown a summary of the different sections where you have made entries.  You can then select to Edit/Add the information from there.  Once done, you would want to go back to the Federal Review tab to check your return. 

No, when I got to the Wages & Income section. it will not let me edit the entry I need to change. In the very beginning  of the return, it asks what changes have been made this year.  I misunderstood and selected "Received Social Security" even though I've been receiving it for several years already.  Now the whole return is treating it like this is something NEW.  How can I undo selecting it as a change for this year?