What's the model number?
Normally you can login to the TP-Link page using the login address and usename/password on the bottom label of a TP-Link device.
In the configuration menu, find wireless to change wireless name and password.
if what you mean is login username and password, go to system tools, then password or administrator.

The TP-Link router comes with a pre-defined username and password : "admin".
Open your browser and type login IP into address bar.Enter 192.168.1.1 into your browser's address bar.If you haven't changed username and password from the factory settings, then they are both 'admin'.If you have changed and no longer remember them,you will have to reset the router.Click wireless,then wireless security.Scroll down and check WPA-PSK/WPA2-PSK box.Type in new password.Click 'save'.Click 'ok'.Click 'system tools'->reboot->ok

Hello
check out this link http://www.tp-link.com/us/faq-73.html that helps you

Click Management->Access Control->Password on left page, and type the old password and newpassword.

Thanks Guys it solved my problem too. I just typed 192.168.1.1 in the url and after that everything was super easy.