Not sure how the title of your posts corresponds to the question within? If you are using a repeater those most the time use WPS to set up. You pretty much just push the buttons and they configure themselves. Just be sure to disable WPS in the router when you are done there is a security exposure if you leave it on.
If you need to manually do it and do not know your password in many cases you will need to reset the router to factory. Of course if you can log into the router you can just key in a new password