You need to login to the wireless router and change the key there to start with. Once you have done that change the key on your computers. Check the users guide for your router and/or the manufacturers support web site for help with the router. Also see this...
http://windows.microsoft.com/en-US/windows7/set-up-a-wireless-router
http://windows.microsoft.com/en-US/windows7/Search?q=wireless&prd=Windows7
My general wireless security guidelines for home users...
http://theillustratednetwork.mvps.org/LAN/SoHoWirelessSecurity.html
MS-MVP Windows Desktop Experience, "When all else fails, read the instructions"