C6300 – AC1750 WiFi Cable Modem Router. i bought used works good . but i cant log in ! to change

What error are you getting? 

did you factory reset it and install it yourself?
To reset, push and hold rest for 7-15 seconds while powered on 

then follow the install guide. 

There's a support site with access to manuals, firmware, install guides, etc.
Go to https://www.netgear.com/support/
Type in your model number and from there it'll take you to the above.
Its a great resource to start with.