Your Wi-Fi password is the first line of defense for your home network. Changing it periodically, especially after having houseguests or after any concern about who has it, is a good practice. Here is how to do it on most routers.
Open a browser and go to your router's admin address. This is usually 192.168.1.1 or 192.168.0.1. If neither works, check the label on the bottom of your router. Log in with your admin username and password (not your Wi-Fi password). If you have never changed it, check the router label for the default credentials.
Look for a section called Wireless, Wi-Fi Settings, or WLAN. If your router has separate 2.4 GHz and 5 GHz bands, you will need to change the password for each one.
Find the field labeled Wi-Fi Password, Network Key, or WPA2 Pre-Shared Key. Enter your new password. Use at least 16 characters, mixing letters, numbers, and symbols. Avoid anything obvious like your address or phone number.
Save the settings. Your router will apply the change and you will be disconnected. Reconnect each of your devices using the new password. This is a good opportunity to notice which devices were on your network and decide which ones actually need to be.
While you are logged in, change the router's admin password if you have never done it. The default admin passwords for most routers are publicly known and leaving them unchanged is a security risk.