Connect to the modems WiFi or with a phone, tablet or PC or connect to a PC via an Ethernet cable plugged in one of the 4 yellow Ethernet ports on the back of the modem. If you don't know the password, you can reset the modem back to default (no WiFi password) and connect without a password. Then open a web browser like Safari, Chrome, Explorer or Mozilla FireFox. Enter 192.168.1.1 in the address bar and press enter. The log in page for modem settings should open. The password is admin.
On the next page click on WiFi to open the WiFi settings.
Next make sure encryption is chosen.
Next enter your desired password and save. After the change is made you will have to reconnect to the WiFi using the new password.