Connect to your router via your browser, by inputting something called a Gateway IP Address.
To find your Gateway IP Address and connect to it in Windows if the IP address is not printed on the bottom of the router.
Click Start > Run > type 'cmd' > Click 'Enter'
Once the Command Prompt window opens, type 'ipconfig /all' and hit 'Enter'
Locate the line labeled 'Gateway' and make note of the number that follows. It will look similar to '192.168.1.1'
Open Internet Explorer (or your favorite browser)
Enter the Gateway IP Address into the address bar and click 'Enter'
You should the get a password box open up. The default name and password is often printed on the bottom of the router, but if it isn't then you will have to look up your router on the internet or try the following default passwords.
Note the name and password is case sensitive. If you cannot log on with the default password you may have to reset the router which may remove your ISP login information. Once logged into the router you can select the security section or wireless section to find and change the wireless password.