by ensuring that she has the router connected to a PC via an ethernet cable and then typing this into her Internet Explorer address bar:-
and pressing the enter key. She will be prompted for a username/password pair, and by default these will both be: admin
If your aunt changed the login details when she set the router up she will obviously have to enter the names she used then. Once into the settings configuration interface your aunt can search for the password settings link, and enter her new password.