You need to follow these steps to change your WiFi password on your MiFi router:
1) Connect to the router wirelessly
2) Open your web browser
3) Enter 192.168.1.1 into the URL bar, or simply browse to mifi.afrihost.com
4) Click on Settings top right, the default username and password are both "admin"
5) Click on WLAN, then Basic Settings
6) Look for a setting called WPA Pre-Shared Key and change it to the new password.
7) Apply the settings. The device will disconnect. Reconnect to it with your new password.