If you dont have any device that is currently connected on your router via wifi, you can use LAN cable (RJ45) to connect to your router,
then you will need to login to router interface.
Please follow the link below on how to login to router interface and retrieve your password on the router interface.
Please note that there is no need to change your password if not necessary, your existing wifi password will be visible on the "Pre shared key/SSID password" field at the bottom of the page.
NB: do note reset your router or make any changes as it will affect your connection.