Below are links on how to change a router's WiFi password

BT - How to change the WiFi password on a BT router

TalkTalk - How to change the WiFi password on a TalkTalk router

Sky -  How to change the WiFi password on a Sky router

Virgin - How to change the WiFi password on a Virgin router

Vodafone - How to change the WiFi password on a Vodafone router

If your Internet Service Provider isn't listed in the above links then use and internet search engine and search for 'Change <ISP name> router WiFi password'