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'