changing device name

> I am trying to change my device name. [...]

   "device name"?  Do you mean the wireless-network name (SSID), or
what?

> [...] I follow the instructions [...]

   _Which_ "the instructions"?  Are you using a web browser, or what?

> [...] use admin for user name and password as default password but it
> doesn't work. [...]

   Which kind of "doesn't work"?  "doesn't work" 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.

   It's possible that someone changed the "admin" password from
"password" to something else.  A settings reset should change it back.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Read.  Look for "Types of
Logins" and "Factory Settings".  Further reading might not hurt, either.
"Manage the Basic WiFi Network Settings", for example.