How do I change the router's "default login credentials" on Netgear Wireless Model AC1900?

> How do I change the default username & password on the Nighthawk
> AC1900??

   "AC1900" is a speed, not a model number.  Ever helpful, Netgear
product packaging emphasizes speeds like "ACxxxx", but that's not the
model identifier.  Look at the product label.

> Model: R6800|Nighthawk AC1900 Smart WiFi Router

   Is that accurate?  Firmware version?

   Which "username & password"?  Visit http://netgear.com/support , put
in your model number, and look for Documentation.  Get the User Manual.
Look for "Types of Logins".  Look for "Change the admin Password".
(Netgear typically doesn't allow changing the "sdmin" user name.)