If your modem is still using defaults, its ip address is 192.168.1.1
I am assuming that you have an HG659 modem, others will be similar.
Log in to your modem using a web-browser and typing its ip address in the address bar, using admin as the user name, and admin as password (check the back of your modem)
Click on "Home Network"
Click on "WLAN Settings"
Click on "WLAN Encryption"
Type a new password in the "WPA pre-shared key" field
(Click on "Show password" tick box if you want to see what password you are typing)
Click o n "Save"
[WOW, this site is censoring some words!]