First thing you gotta do is to make sure that your computer/laptop is connected directly to the modem via ethernet/RJ-45 cable aka yellow/blue cable. You can also use a tablet or smartphone provided that it's conmected wirelessly to your modem. Open up an internet browser. Any browser such as Internet Explorer, Google Chrome, Safari, Mozilla Firefox will do. In my case, I used my iPad so I used safari.
On the address bar or URL bar (on this browser, it's the search bar at the very top) type in 192.168.100.1 (the default IP address of this modem) then press enter/return. It will go to a page that looks like this.
By default, the account is "root" and the password is "admin". After you type the info, click on login and it will go to a page that looks like this.
Just click/tap the WLAN tab and it will go to the WLAN Configuration page.
By default, the SSID aka wifi name is set to WirelessNet and it doesn't have a password. So just clear out the SSID name box and type in the name of wifi that you wanna use. Remember, wifi name should not have any space. To customize your wifi password, first, you have to select an Authentication mode. I recommend using WPA/WPA2 coz it's more secure than WEP plus you can use both letters and numbers for the password unlike in WEP, wherein you can only use numbers. Once you have selected WPA/WPA2, another box below Authentication Mode that says Encryption mode will pop up. Just select TKIP&AES then type your preferred password on the password box which should have at least a minimum of 8 characters (includes letters and numbers). Just click on apply and you're all set!
Depending on how your device is connected, it may or may not get disconnected after you click on the apply button. If your pc/laptop is hardwired (connected via ethernet cable) to the modem, it will not get disconnected but if you're gonna use a pc/laptop/tablet/smartphone that is connected wirelessly, your device will definitely get disconnected from wifi since you changed the wifi name and password. All you have to do is to reconnect wirelessly. Just search for your wifi name, click connect and you'll be prompted to enter the wifi password. Just enter the new wifi password and you'll be good to go!
Hope this helps! If you have other questions, feel free to comment and I'll try my best to answer it.
God Bless and Have a good day!