follw these steps
1.go to control panel
2.click network connections
3.right click the wireless network card you use to connect to the wireless network
4.click the properties
5.you will see the three menus on the top and click the wireless menu
6.this will bring up all the wireless ssids you have been using
7.find the one you want to use now,click that and then properties and this will bring up all the setup for that connection
8.now change whatever you want to do.
(please remember there are different encryption methods like wep wap wap2 etc.check whether the serect key has AES OR TKIP)
there are many ways to reach upto step 6 follow whatever you know or find it convenient,i have given the regular standard way
Good luck