Retrieving My Netgear C6300 admin/username and password

> How do I reset or retrieve [...]

   "retrieve" is not practical.  For "reset", you might try this radical
concept:

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

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual (at least).  Read.  Look for
"Reset" and/or "Factory Default Settings".

> [...] Admin and password do not allow me to log in [...]

   "Admin" or "admin"?

   "not allow" is not a useful problem description.  It does not say
what you did.  It does not say what happened when you did it.  As usual,
showing actual actions (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

   A clear description of what _does_ happen would be more useful
than a vague description of what does _not_ happen.