How can I change security password on a Nighthawk R7000 Router I purchased used

> Model: A7000|Nighthawk AC1900 WiFi USB Adapter - USB 3.0

   Nothing to do with anything?

> How can I change security password on a Nighthawk R7000 Router I
> purchased used

   Which password is "security password"?  Which password is _not_ a
"security password"?

   You might try this radical concept which works for some people:

      https://en.wikipedia.org/wiki/RTFM

   Visit http://netgear.com/support , put in your (actual) model number,
and look for Documentation.  Get the User Manual (at least).  Read.
Look for "Types of Logins" and "Reset".  And "Specify Basic WiFi
Settings".  Further reading might not hurt, either.