On the Control Panel, click on the WiFi tab and scroll down the page to the Private Broadcast section. On the line labeled Password (WPA Key), insert your desired password. Click Save Changes on the lower left corner of the page. Once Save Changes has finished, your computers / devices will be disconnected from the broadcast of the WiFiRanger as the password has now changed. Reconnect your computers / devices using the new password.
In some cases, you may need to forget the stored password for the WiFiRanger network from your computer / device as it may keep trying to use the original password, which of course will no longer work.