@Islami 

Hi, may I know which password you have forgotten?

If it is Wi-Fi password, the extender shares the same Wi-Fi password as the router. You can try router's Wi-Fi password to connect the extender's network.

If it is login password to login the management page of the extender, you can try with default username/password admin in lower case to check if it works.

If it doesn't work, it means that you have created your own password to login this page.

Then, you may need to reset it to default factory settings first, reconfigure the extender again to make it works well.

Note: Use a pin to press the reset button for about 5 seconds until all the lights blink once to reset it.

You may also need the instruction to set it up again. https://www.tp-link.com/support/faq/1400/

Hope this answer will help you.

Have a nice day!