IF you haven't installed the software included in the wifi, plug it in first in your computer then wait for the auto play. After seeing the autoplay, choose install. After installing, click the program (or the shortcut on your desktop).
The default password is "admin" for the log in page. Once you are logged in, click on the settings, then "WLAN" now choose the basic settings. There you will see the options in which will allow you to customize the settings of your wifi. Especially, your main concern, the password. Once you have written your chosen password, click apply. Your devices will be disconnected from your wifi and you have to reconnect it with your new password.
(btw, same procedure happens when you want to change your wifi's name)
Hope this helps!
1. Determine the admin interface for your router and enter it in your browser
2.Log in to the administrative console on the router
3. Under the wireless security configuration section, ensure that you are using the latest encryption available
4. Create a strong password for the Pre Shared Key (wireless network password)