Network routers are managed through a special administrative account. Router manufacturers set a default username and default password for this account at the factory, generally using the same settings for all of their equipment. To improve the security of your network, you should change the administrative password immediately after installing the unit.
These pages walk through the steps to change the default password on a common Linksys network router. The exact steps will vary depending on the specific model of router in use, but the process is similar in any case.