How do I revert from 2-column layout to 1-column layout ?

I have an RCA Viking 10" tablet. I use it to read ePub books with the Adobe Digital Editions app.

All was well until . . . . . I seem to have inadvertently changed the layout from 1-column to 2-column.

I don't know how this happened. How can I get it back to 1-column layout ?  On the same subject,

I try viewing the menu by dragging over from the upper left hand area of screen.  I see the following

menu items:  Library, Cover Page, Contents, Go To Page, Bookmarks, Highlights. 

My question is, where are "Settings" ???  Shouldn't there be another menu item called "Settings" ??

I do not believe this is tablet-brand (RCA) specific. I believe this is specific to the Adobe DE app

which should work the same (at least in this regard) on all Android devices.

I checked the online help for Adobe DE (not very well produced - i.e., not very helpful) and it led me

to this page: Adobe Digital Publishing Solution‌ which does not have any RCA devices listed.