First, note that you must be logged into an account with administrative access. For details on this subject, refer to this article.
To adjust a day open or close, click Retail Chain from the top bar, and click Day Openings and Closings from the submenu.
From this screen, you can add a closing/opening (by the New button at the top, or the plus icon at the bottom), delete a closing/opening (by the X icon to the right of each entry), or modify a closing/opening (by clicking on the particular entry).
Clicking on an entry will bring you to this screen, in which you can manipulate the attributes of a given open/close.