If you've forgotten it, then you'll have to do a hard reset on your router and try and force it to forget it's settings and go back to default.
Some routers are easily changed, some you have to play around until the settings have changed.
Best way to proceed. Get the manual for the router first, it may tell you the technique (if you don't have the manual, download it from the manufacturers website).
Otherwise, do a websearch for "resetting [model number] router". See what you can find.