You would need to login to your router and change it. Its usually located at http/192.168.0.1, .0.100, or 0.0 Here is how to find your default gateway: http/kb.iu.edu/data/ajfx.html
You can login - default is sometimes admin for the username and password for the password (or admin again sometimes). Look up your router's instruction manual it might have some of this information. What type of router do you have?