Browse to http://192.168.1.1 (this is the default IP address of your router) on a computer or device that is currently connected to your router (either wired or wirelessly).
The router login screen appears. Enter "admin" as the user name and its password as printed on the product label of your router. The router's web interface appears.
Click on the "Basic" tab follows by Wireless Settings in the LH margin. The Wireless Settings page appears.
At or near near the foot of the page you will see in the Wireless Password box the default WiFi password in clear as printed on the product label of your router. Here overtype the new wireless password that you want to use. The password must consist of 8 to 63 alphanumeric characters or minus ('-').
For a Brightbox 2 you will see the Wireless Password box repeated under both the 2.4 & 5 GHz Settings. Change both of them. They don't need to be the same but usually are.
Reconnect your wireless devices to the router using the new wireless settings.
If you have the TG589 router, you can find the equivalent steps covered in Configuring your TG589 router services.
It may be wise to also change the router admin password by following Change admin password.
If you ever factory reset your router, the default WiFi password will come back into play. So don't forget to change it back to your chosen password.