The method for changing your password varies based on the hub you use. You have to look in your hub/router's user guide for directions on how to log into the device. I think for British Telecom, you type into your browser, http://bthomehub.home
You then need to find the WPA or other encryption settings to change your access code. If you change your password you, obviously, need to tell Windows or whatever you use to connect with the new password. Alternatively, you can change your settings to allow only computers with authorized MAC addresses (Think of a MAC address as a fingerprint, every connected device has a unique MAC address readable by the hub) to connect. Unauthorized devices cannot connect even if they have the password to log in. Using MAC filtering will slow your connection. You can also change your SSID, the name your hub transmits to identify your network, to prevent access since a user needs to know what network they are connecting to before they can use it. But if you live in a rural area without several nearby networks, it will be easy for your neighbor to determine what your new SSID is since there are a limited number of them.