What make/model of router are you using? With that info, you can find the specific manual/guide on the manufacturer website to walk you through the process.
Typically you go to 192.168.1.1 (this is the IP address used by many routers) and use your admin login to change the wireless settings. You would also have to change the settings on your connected devices (PC, smart phone,etc) afterward so that they too can connect to the network with the updated configuration.