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.
What actual extender do you have?
did you hit the wps button? if so, it could be already setup using the same ssid as the router. That's part of it being a mesh extender.
If so, you could try factory resetting it by pushing and holding reset for 7-15 seconds while powered on.
