|If you are referring to the admin / login pasword to access the router management and setup then that's usually admin / password.|
If the wifi aspect, then you login to the homepage, using the above (default) details and then go to the wifi setup section.
If the default login etc has changed for homepage access, reset the router to factory defaults. This is usually via small hole in the rear panel, or less often a wee button on the rear.
This reset wipes all previous user settings, changes etc. restoring it as delivered when new.
This link - one of many on this theme - may help further.