Changed WiFi router how to get extender to connect to new router

I'd factory reset the extender (push and hold reset 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.