Do you have a computer that's already connected to that wifi? If so, do the following:
1. Click Start and type Command Prompt. Open Command Prompt
2. Type "ipconfig" and press Enter
3. Look for Default Gateway's IP address (e.g. 192.168.1.1). Remember that and go back to your browser.
4. Input the IP address on your browser's address bar
5. You should now be on the homepage of your wifi modem-router's settings.
6. Go to the WiFi settings and change the wifi password. If prompted by a system password, please input it or refer to your device's stickers or manual for the default system password.
7. As soon as you hit Save, your computer will be disconnected and so would be other devices previously connected to it. Your browser page will time out or will be unresponsive. If prompted by a system password, please input it or refer to your device's stickers or manual for the default system password.
8. Forget the old wifi network (Instructions vary per OS).
9. Reconnect to the wifi using the new password.
To skip step 8, it's best that you change the wifi name na rin.