The homepage for nextbook tablets is
Assuming that your tablet is WORKING, the first important thing to do is to identify the Model Number. "How do I do that?", I hear you ask. Simple. Look for the writing on the back of the tablet. If that is illegible (I think tablet manufacturers should face some penalties for failure to properly label their products: but that's another issue), .... if illegible, I say, then you need to look in the "Settings- About Tablet" information stored on your tablet.
A word of caution: be OBSERVANT. Those "SETTINGS" menu (and sub-menu) items may be "hidden" depending on the version of the operating system installed. So let observation be your guide. But trust me: the MODEL and the OPERATING SYSTEM specifics are stored THERE!
Once you have that information, go visit the NEXTBOOK website. Specifically go to http://www.nextbookusa.com/download.php
and use the pull-down menus you find there to locate the manual you need for your NEXTBOOK:. It will probably be a PDF document. Then you are home free. I hope this helps.