On the bottom of the gateway / modem should be the 'admin' password, and the IP address of the gateway. Once you go to the IP address (usually 192.168.1.254) in your browser, a web page appears. Login using the admin and password, and on the page describing Wireless, should be the option to change your Wi-Fi password to one more memorable.
Tell us which gateway you are using, and we can provide more info.
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