You need your router's password to change settings or beef up security. It's easy to find if you have the manual, but can be a tough job if you don't.
This site offers thousands of default passwords for routers. Most of the popular models can be found, and there are even some out-of-production routers if you buy used. Just search for the manufacturer and then the model number.
Now, this site should remind you of something. If it's this easy for you to find your router's password - how easy is it for a hacker? As soon as you figure out what your password is, make sure you change it to something that isn't as easy to guess.