The WiFi password needs to be changed on the WiFi router (or whatever device provides WiFi for your home).
How this is done is different for each make and model WiFi router.
If this is an Apple Airport Extreme, Airport Express or Time Capsule, then look at Applications -> Utilities -> Airport Utility
If the router is from your ISP, then you may be able to call them and get them to change it for you.
Other wise you generally fire up a web browser (such as Safari) and use the WiFi router's local IP address, such as 192.168.1.1 (but a lot depends on what the router uses as its address; eg. 192.168.1.1 or 192.168.2.1 or 192.168.3.1 or 10.0.0.1 or 10.0.1.1 or 172.16.1.1 etc...)
It is best if you actually find the WiFi router's manuals online and read how they change the WiFi password, and what the default WiFi route's IP address is.